refactor(client): wrap all external requests in retry transport

.env.sample

@@ -3,6 +3,7 @@ ZARINHUB_API_PASSWORD=
 ZARINHUB_AUTH_ENDPOINT=https://zarin-hub.com/api/v5/Authentication/GetToken
 ZARINHUB_ENDPOINT=https://zarin-hub.com/api/v5/KYC/CardToIban
 ZARINHUB_TIMEOUT=5s
+ZARINHUB_RETRIES=2
 
 DATABASE_PATH=./card-to-iban.db
 SERVER_ADDR=127.0.0.1:8080

README.md

@@ -1,92 +1,59 @@
 # Card to IBAN
 
-A small full-stack application that converts an Iranian bank card number to its IBAN using the ZarinHub API. The backend is written in Go with the standard HTTP library, while the frontend uses Nuxt, Vue, and Tailwind CSS.
+A small full-stack application that converts an Iranian bank card number to its IBAN through ZarinHub.
 
-The request flow is intentionally simple:
+- Backend: Go, `net/http`, SQLite
+- Frontend: Nuxt, Vue, Tailwind CSS
+- API documentation: OpenAPI 3.0 and Swagger UI
 
-```text
-Nuxt UI -> HTTP handler -> Card service -> ZarinHub client
-                    |
-                    -> Audit repository -> SQLite
-```
+## Quick start with Docker
 
-The HTTP handler coordinates the public API contract and audit persistence. The service validates card numbers, the ZarinHub client handles external requests, and the repository contains the SQLite-specific implementation.
-
-## Prerequisites
-
-- Go 1.27 or later
-- Node.js 22 or later
-- ZarinHub API credentials
-
-## Configuration
-
-Copy the example environment file from the project root:
+Copy the environment template and add your ZarinHub API credentials:
 
 ```powershell
 Copy-Item .env.sample .env
 ```
 
-Set `ZARINHUB_API_USERNAME` and `ZARINHUB_API_PASSWORD` in the new `.env` file. These are the API credentials created in the ZarinHub panel, not the regular panel login credentials. The `.env` file is ignored by Git.
-
-| Variable | Default | Purpose |
-| --- | --- | --- |
-| `ZARINHUB_API_USERNAME` | None; required | API username created in the ZarinHub panel |
-| `ZARINHUB_API_PASSWORD` | None; required | API password created in the ZarinHub panel |
-| `ZARINHUB_AUTH_ENDPOINT` | Official GetToken endpoint | Token acquisition and refresh endpoint |
-| `ZARINHUB_ENDPOINT` | Official CardToIban endpoint | External service endpoint |
-| `ZARINHUB_TIMEOUT` | `5s` | External request timeout |
-| `DATABASE_PATH` | `./card-to-iban.db` | SQLite database file path |
-| `SERVER_ADDR` | `127.0.0.1:8080` | Backend HTTP address |
-| `FRONTEND_ORIGIN` | `http://127.0.0.1:3000` | Allowed frontend origin |
-
-The frontend reads the following value from `frontend/.env` and uses the shown address by default:
-
 ```env
-NUXT_PUBLIC_API_BASE_URL=http://127.0.0.1:8080
+ZARINHUB_API_USERNAME=your-api-username
+ZARINHUB_API_PASSWORD=your-api-password
 ```
 
-The backend obtains an access token during startup, keeps the access and refresh tokens in memory, and refreshes them automatically shortly before expiration. Credentials and tokens are never written to the database or logs.
+Build and start the application:
 
-## Running the Project
+```powershell
+docker compose up --build
+```
 
-Start the backend from the Go module directory. The root `.env` file is loaded automatically, and the database migration runs during startup:
+Open:
+
+- App: `http://127.0.0.1:3000`
+- Health check: `http://127.0.0.1:8080/health`
+- Swagger UI: `http://127.0.0.1:8080/docs/`
+- OpenAPI document: `http://127.0.0.1:8080/openapi.json`
+
+After backend changes, restart it or run `docker compose up --build` again so the running container uses the latest code.
+
+## Local development
+
+Start the backend:
 
 ```powershell
 cd backend
 go run ./cmd/api
 ```
 
-In another terminal, install the frontend dependencies and start the development server:
+Start the frontend in another terminal:
 
 ```powershell
 cd frontend
-npm install
+npm ci
 npm run dev
 ```
 
-The frontend is available at `http://127.0.0.1:3000`. The backend health endpoint is available at `http://127.0.0.1:8080/health`.
+The backend loads `.env` from the project root. The frontend uses `http://127.0.0.1:8080` by default; override it with `NUXT_PUBLIC_API_BASE_URL` when needed.
 
-## Running with Docker Compose
-
-Create the root `.env` file as described in the configuration section, then build and start both services:
-
-```powershell
-docker compose up --build
-```
-
-Open `http://localhost:3000` in your browser. The API is exposed at `http://localhost:8080`, and SQLite data is kept in the named `card_to_iban_data` volume.
-
-Stop the services with:
-
-```powershell
-docker compose down
-```
-
-The database volume is preserved by default. To remove it intentionally, run `docker compose down --volumes`.
-
-## API Contract
-
-### Convert a card number to an IBAN
+## API
 
 ```http
 POST /api/v1/card-to-iban
@@ -106,44 +73,17 @@ Successful response:
 }
 ```
 
-All errors use the same response structure. Error messages are returned in Persian to match the user interface:
+Errors use the same `requestId` returned in the `X-Request-ID` header. See Swagger UI for the complete request, response, and error contract.
 
-```json
-{
-  "error": {
-    "code": "invalid_card",
-    "message": "شماره کارت باید شامل ۱۶ رقم باشد."
-  },
-  "requestId": "SERVER_GENERATED_REQUEST_ID"
-}
-```
+## Implementation notes
 
-The same request ID is also returned in the `X-Request-ID` response header.
+- The backend validates card numbers and refreshes ZarinHub access tokens automatically.
+- Every conversion attempt is audited in SQLite.
+- Card numbers and upstream responses are sanitized before storage; credentials and tokens are not persisted.
+- Unknown frontend routes display a branded 404 page with a link back to the application.
+- SQLite data is stored in the `card_to_iban_data` Docker volume when using Compose.
 
-| HTTP status | Stable error code | Meaning |
-| --- | --- | --- |
-| `400` | `invalid_card` | The JSON body or card number format is invalid |
-| `405` | `method_not_allowed` | The request method is not `POST` |
-| `422` | `conversion_rejected` | ZarinHub rejected the conversion |
-| `502` | `upstream_failure` | ZarinHub authentication failed or its response was invalid |
-| `503` | `upstream_unavailable` | The upstream service is temporarily unavailable |
-| `504` | `upstream_timeout` | The upstream request timed out |
-| `500` | `internal_error` | Audit persistence or another internal operation failed |
-
-## Auditing and Data Security
-
-- Every conversion attempt creates one audit record, including invalid requests and upstream failures.
-- A response is returned only after its audit record is saved successfully. A database failure produces a generic `500` response.
-- Full card numbers are never written to the database or logs. Stored values are masked, for example `603799******5678`.
-- ZarinHub JSON responses are sanitized recursively before storage, and every field named `card` is masked.
-- Non-JSON upstream responses are not stored because their contents cannot be safely verified.
-- Client IP addresses are intentionally not collected, so the related audit column remains empty.
-- Structured logs contain only the request ID, route, HTTP status, audit outcome, and duration.
-- Request bodies are limited to 1 KiB, and ZarinHub response bodies are limited to 1 MiB.
-
-## Testing
-
-Backend tests do not use real credentials. They use fakes and `httptest` for ZarinHub requests:
+## Tests
 
 ```powershell
 cd backend
@@ -151,21 +91,8 @@ go test ./...
 go vet ./...
 ```
 
-Run the frontend tests and production build with:
-
 ```powershell
 cd frontend
 npm test
 npm run build
 ```
-
-The test suite covers card validation and masking, the conversion service, the ZarinHub client, response-size limits, the SQLite repository, JSON sanitization, HTTP outcomes, audit failures, Persian and Arabic digit normalization, and frontend success and error states.
-
-## Assumptions and Limitations
-
-- Backend validation intentionally accepts only exactly 16 ASCII digits. Luhn validation is outside the assessment requirements.
-- The frontend normalizes Persian and Arabic digits and removes spaces or hyphens before submission.
-- SQLite is sufficient for this assessment and uses a single writer connection for predictable behavior.
-- Only the IBAN is exposed to the frontend. Account details, owner information, and raw ZarinHub responses remain internal.
-- Retries, Docker, OpenAPI, CI, dark mode, and production infrastructure are intentionally outside the project scope.
-- The interface uses the existing ZarinPal design system's font and logo assets. The font license is stored in `frontend/public/fonts`.

backend/cmd/api/config.go

@@ -3,6 +3,7 @@ package main
 import (
 	"fmt"
 	"os"
+	"strconv"
 	"strings"
 	"time"
 )
@@ -15,6 +16,7 @@ type config struct {
 	ZarinHubUsername     string
 	ZarinHubPassword     string
 	ZarinHubTimeout      time.Duration
+	ZarinHubRetries      int
 	DatabasePath         string
 }
 
@@ -44,6 +46,18 @@ func readConfig(getenv func(string) string) (config, error) {
 		)
 	}
 
+	retriesText := valueOrDefault(
+		getenv("ZARINHUB_RETRIES"),
+		"2",
+	)
+
+	retries, err := strconv.Atoi(retriesText)
+	if err != nil || retries < 0 {
+		return config{}, fmt.Errorf(
+			"ZARINHUB_RETRIES must be a non-negative integer",
+		)
+	}
+
 	return config{
 		ServerAddr: valueOrDefault(
 			getenv("SERVER_ADDR"),
@@ -64,6 +78,7 @@ func readConfig(getenv func(string) string) (config, error) {
 		ZarinHubUsername: username,
 		ZarinHubPassword: password,
 		ZarinHubTimeout:  timeout,
+		ZarinHubRetries: retries,
 		DatabasePath: valueOrDefault(
 			getenv("DATABASE_PATH"),
 			"./card-to-iban.db",

backend/cmd/api/config_test.go

@@ -31,6 +31,9 @@ func TestReadConfigUsesDefaults(t *testing.T) {
 	if config.ZarinHubTimeout != 5*time.Second {
 		t.Errorf("ZarinHubTimeout = %v", config.ZarinHubTimeout)
 	}
+	if config.ZarinHubRetries != 2 {
+		t.Errorf("ZarinHubRetries = %d, want 2", config.ZarinHubRetries)
+	}
 	if config.DatabasePath != "./card-to-iban.db" {
 		t.Errorf("DatabasePath = %q", config.DatabasePath)
 	}
@@ -70,3 +73,22 @@ func TestReadConfigRejectsInvalidTimeout(t *testing.T) {
 		t.Fatalf("readConfig() error = %v, want timeout error", err)
 	}
 }
+
+func TestReadConfigRejectsInvalidRetries(t *testing.T) {
+	testCases := []string{"-1", "invalid", "2.5"}
+
+	for _, tc := range testCases {
+		values := map[string]string{
+			"ZARINHUB_API_USERNAME": "test-username",
+			"ZARINHUB_API_PASSWORD": "test-password",
+			"ZARINHUB_RETRIES":      tc,
+		}
+
+		_, err := readConfig(func(key string) string {
+			return values[key]
+		})
+		if err == nil || !strings.Contains(err.Error(), "ZARINHUB_RETRIES") {
+			t.Errorf("retries=%q: want error mentioning ZARINHUB_RETRIES, got %v", tc, err)
+		}
+	}
+}

backend/cmd/api/main.go

@@ -38,7 +38,11 @@ func run(logger *slog.Logger) error {
 		return fmt.Errorf("load configuration: %w", err)
 	}
 
-	httpClient := &http.Client{Timeout: config.ZarinHubTimeout}
+	httpClient := zarinhub.NewHTTPClient(
+		config.ZarinHubTimeout,
+		config.ZarinHubRetries,
+		logger,
+	)
 	tokenManager := zarinhub.NewTokenManager(
 		httpClient,
 		config.ZarinHubAuthEndpoint,
@@ -46,9 +50,10 @@ func run(logger *slog.Logger) error {
 		config.ZarinHubPassword,
 	)
 
+	authTimeout := time.Duration(config.ZarinHubRetries+1)*config.ZarinHubTimeout + 10*time.Second
 	authContext, cancelAuth := context.WithTimeout(
 		context.Background(),
-		config.ZarinHubTimeout,
+		authTimeout,
 	)
 	_, err = tokenManager.AccessToken(authContext)
 	cancelAuth()
@@ -79,6 +84,8 @@ func run(logger *slog.Logger) error {
 		httpClient,
 		config.ZarinHubEndpoint,
 		tokenManager,
+		config.ZarinHubRetries,
+		logger,
 	)
 	cardService := service.NewCardService(zarinHubClient)
 	auditRepository := repository.NewAuditRepository(db)
@@ -91,6 +98,7 @@ func run(logger *slog.Logger) error {
 	mux := http.NewServeMux()
 	mux.HandleFunc("/health", healthHandler)
 	mux.HandleFunc("/api/v1/card-to-iban", handler.Convert)
+	httpapi.RegisterDocumentation(mux)
 
 	server := &http.Server{
 		Addr: config.ServerAddr,

backend/internal/httpapi/client_ip.go

@@ -0,0 +1,63 @@
+package httpapi
+
+import (
+	"net"
+	"net/http"
+	"strings"
+)
+
+func extractClientIP(r *http.Request) *string {
+	if r == nil {
+		return nil
+	}
+
+	// Priority headers commonly set by reverse proxies, load balancers, and CDNs.
+	headers := []string{
+		"CF-Connecting-IP", // Cloudflare
+		"True-Client-IP",   // Cloudflare Enterprise, Akamai
+		"X-Real-IP",        // Nginx, Traefik
+	}
+
+	for _, header := range headers {
+		if val := strings.TrimSpace(r.Header.Get(header)); val != "" {
+			if ip := parseIP(val); ip != "" {
+				return &ip
+			}
+		}
+	}
+
+	// X-Forwarded-For can contain multiple comma-separated IPs: client, proxy1, proxy2. The original client IP is the first entry.
+	if xff := r.Header.Get("X-Forwarded-For"); xff != "" {
+		for _, part := range strings.Split(xff, ",") {
+			if ip := parseIP(part); ip != "" {
+				return &ip
+			}
+		}
+	}
+
+	// Fallback to RemoteAddr (typically "host:port" or "[ipv6]:port").
+	if r.RemoteAddr != "" {
+		host, _, err := net.SplitHostPort(r.RemoteAddr)
+		if err != nil {
+			host = r.RemoteAddr
+		}
+		if ip := parseIP(host); ip != "" {
+			return &ip
+		}
+	}
+
+	return nil
+}
+
+func parseIP(raw string) string {
+	raw = strings.TrimSpace(raw)
+	raw = strings.TrimPrefix(raw, "[")
+	raw = strings.TrimSuffix(raw, "]")
+
+	ip := net.ParseIP(raw)
+	if ip == nil {
+		return ""
+	}
+
+	return ip.String()
+}

backend/internal/httpapi/client_ip_test.go

@@ -0,0 +1,143 @@
+package httpapi
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+func TestExtractClientIP(t *testing.T) {
+	tests := []struct {
+		name       string
+		request    *http.Request
+		headers    map[string]string
+		remoteAddr string
+		wantIP     *string
+	}{
+		{
+			name:    "nil request",
+			request: nil,
+			wantIP:  nil,
+		},
+		{
+			name: "CF-Connecting-IP takes precedence",
+			headers: map[string]string{
+				"CF-Connecting-IP": "203.0.113.10",
+				"X-Real-IP":        "198.51.100.20",
+				"X-Forwarded-For":  "192.0.2.30",
+			},
+			remoteAddr: "10.0.0.1:54321",
+			wantIP:     ptr("203.0.113.10"),
+		},
+		{
+			name: "True-Client-IP takes precedence over X-Real-IP",
+			headers: map[string]string{
+				"True-Client-IP":  "203.0.113.11",
+				"X-Real-IP":       "198.51.100.20",
+				"X-Forwarded-For": "192.0.2.30",
+			},
+			remoteAddr: "10.0.0.1:54321",
+			wantIP:     ptr("203.0.113.11"),
+		},
+		{
+			name: "X-Real-IP takes precedence over X-Forwarded-For",
+			headers: map[string]string{
+				"X-Real-IP":       "198.51.100.20",
+				"X-Forwarded-For": "192.0.2.30",
+			},
+			remoteAddr: "10.0.0.1:54321",
+			wantIP:     ptr("198.51.100.20"),
+		},
+		{
+			name: "X-Forwarded-For first entry is extracted",
+			headers: map[string]string{
+				"X-Forwarded-For": "192.0.2.30, 10.0.0.2, 172.16.0.5",
+			},
+			remoteAddr: "10.0.0.1:54321",
+			wantIP:     ptr("192.0.2.30"),
+		},
+		{
+			name: "X-Forwarded-For single entry",
+			headers: map[string]string{
+				"X-Forwarded-For": " 192.0.2.31 ",
+			},
+			remoteAddr: "10.0.0.1:54321",
+			wantIP:     ptr("192.0.2.31"),
+		},
+		{
+			name: "X-Forwarded-For skips invalid first entry to find valid IP",
+			headers: map[string]string{
+				"X-Forwarded-For": "unknown, 192.0.2.32",
+			},
+			remoteAddr: "10.0.0.1:54321",
+			wantIP:     ptr("192.0.2.32"),
+		},
+		{
+			name: "Fallback to RemoteAddr with port",
+			headers: map[string]string{},
+			remoteAddr: "192.168.1.100:12345",
+			wantIP:     ptr("192.168.1.100"),
+		},
+		{
+			name: "Fallback to RemoteAddr without port",
+			headers: map[string]string{},
+			remoteAddr: "192.168.1.101",
+			wantIP:     ptr("192.168.1.101"),
+		},
+		{
+			name: "IPv6 in RemoteAddr with port",
+			headers: map[string]string{},
+			remoteAddr: "[2001:db8::1]:8080",
+			wantIP:     ptr("2001:db8::1"),
+		},
+		{
+			name: "IPv6 in header",
+			headers: map[string]string{
+				"X-Real-IP": "2001:db8::2",
+			},
+			remoteAddr: "10.0.0.1:54321",
+			wantIP:     ptr("2001:db8::2"),
+		},
+		{
+			name: "Invalid headers fall back to RemoteAddr",
+			headers: map[string]string{
+				"X-Real-IP":       "not-an-ip",
+				"X-Forwarded-For": "also-not-an-ip",
+			},
+			remoteAddr: "192.168.1.102:443",
+			wantIP:     ptr("192.168.1.102"),
+		},
+		{
+			name: "No headers and empty RemoteAddr",
+			headers: map[string]string{},
+			remoteAddr: "",
+			wantIP:     nil,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var req *http.Request
+			if tt.name != "nil request" {
+				req = httptest.NewRequest(http.MethodGet, "/", nil)
+				for k, v := range tt.headers {
+					req.Header.Set(k, v)
+				}
+				req.RemoteAddr = tt.remoteAddr
+			}
+
+			got := extractClientIP(req)
+			if (got == nil) != (tt.wantIP == nil) {
+				t.Fatalf("extractClientIP() = %v, want %v", got, tt.wantIP)
+			}
+			if got != nil && tt.wantIP != nil && *got != *tt.wantIP {
+				t.Errorf("extractClientIP() = %q, want %q", *got, *tt.wantIP)
+			}
+		})
+	}
+}
+
+func ptr(s string) *string {
+	return &s
+}
+

backend/internal/httpapi/docs.go

@@ -0,0 +1,53 @@
+package httpapi
+
+import (
+	_ "embed"
+	"net/http"
+)
+
+//go:embed openapi.json
+var openAPIDocument []byte
+
+const swaggerUI = `<!doctype html>
+<html lang="en">
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1">
+  <title>Card to IBAN API documentation</title>
+  <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/swagger-ui-dist@5/swagger-ui.css">
+</head>
+<body>
+  <div id="swagger-ui"></div>
+  <script src="https://cdn.jsdelivr.net/npm/swagger-ui-dist@5/swagger-ui-bundle.js"></script>
+  <script>
+    window.onload = () => SwaggerUIBundle({
+      url: "/openapi.json",
+      dom_id: "#swagger-ui",
+      deepLinking: true,
+      presets: [SwaggerUIBundle.presets.apis]
+    });
+  </script>
+</body>
+</html>
+`
+
+// RegisterDocumentation registers the OpenAPI document and its Swagger UI.
+func RegisterDocumentation(mux *http.ServeMux) {
+	mux.HandleFunc("GET /openapi.json", serveOpenAPI)
+	mux.HandleFunc("GET /docs/", serveSwaggerUI)
+	mux.HandleFunc("GET /docs", redirectToDocumentation)
+}
+
+func serveOpenAPI(w http.ResponseWriter, _ *http.Request) {
+	w.Header().Set("Content-Type", "application/json")
+	_, _ = w.Write(openAPIDocument)
+}
+
+func serveSwaggerUI(w http.ResponseWriter, _ *http.Request) {
+	w.Header().Set("Content-Type", "text/html; charset=utf-8")
+	_, _ = w.Write([]byte(swaggerUI))
+}
+
+func redirectToDocumentation(w http.ResponseWriter, r *http.Request) {
+	http.Redirect(w, r, "/docs/", http.StatusTemporaryRedirect)
+}

backend/internal/httpapi/docs_test.go

@@ -0,0 +1,124 @@
+package httpapi
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+func TestRegisterDocumentation(t *testing.T) {
+	tests := []struct {
+		name            string
+		path            string
+		wantStatus      int
+		wantContentType string
+		wantBody        string
+		wantLocation    string
+	}{
+		{
+			name:            "OpenAPI document",
+			path:            "/openapi.json",
+			wantStatus:      http.StatusOK,
+			wantContentType: "application/json",
+			wantBody:        `"openapi": "3.0.3"`,
+		},
+		{
+			name:            "Swagger UI",
+			path:            "/docs/",
+			wantStatus:      http.StatusOK,
+			wantContentType: "text/html; charset=utf-8",
+			wantBody:        `url: "/openapi.json"`,
+		},
+		{
+			name:         "documentation trailing slash",
+			path:         "/docs",
+			wantStatus:   http.StatusTemporaryRedirect,
+			wantLocation: "/docs/",
+		},
+		{
+			name:       "removed Swagger route",
+			path:       "/swagger/",
+			wantStatus: http.StatusNotFound,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			mux := http.NewServeMux()
+			RegisterDocumentation(mux)
+			request := httptest.NewRequest(http.MethodGet, tt.path, nil)
+			response := httptest.NewRecorder()
+
+			mux.ServeHTTP(response, request)
+
+			if response.Code != tt.wantStatus {
+				t.Fatalf("status = %d, want %d", response.Code, tt.wantStatus)
+			}
+			if tt.wantContentType != "" {
+				if got := response.Header().Get("Content-Type"); got != tt.wantContentType {
+					t.Errorf("Content-Type = %q, want %q", got, tt.wantContentType)
+				}
+			}
+			if tt.wantBody != "" && !strings.Contains(response.Body.String(), tt.wantBody) {
+				t.Errorf("body does not contain %q", tt.wantBody)
+			}
+			if got := response.Header().Get("Location"); got != tt.wantLocation {
+				t.Errorf("Location = %q, want %q", got, tt.wantLocation)
+			}
+		})
+	}
+}
+
+func TestOpenAPIDocumentCoversPublicAPI(t *testing.T) {
+	t.Parallel()
+
+	mux := http.NewServeMux()
+	RegisterDocumentation(mux)
+	request := httptest.NewRequest(http.MethodGet, "/openapi.json", nil)
+	response := httptest.NewRecorder()
+	mux.ServeHTTP(response, request)
+
+	var document struct {
+		OpenAPI string `json:"openapi"`
+		Paths   map[string]map[string]struct {
+			Responses map[string]json.RawMessage `json:"responses"`
+		} `json:"paths"`
+		Components struct {
+			Schemas map[string]json.RawMessage `json:"schemas"`
+		} `json:"components"`
+	}
+	if err := json.Unmarshal(response.Body.Bytes(), &document); err != nil {
+		t.Fatalf("decode OpenAPI document: %v", err)
+	}
+
+	if document.OpenAPI != "3.0.3" {
+		t.Errorf("OpenAPI version = %q, want 3.0.3", document.OpenAPI)
+	}
+	if _, ok := document.Paths["/health"]["get"]; !ok {
+		t.Error("OpenAPI document does not define GET /health")
+	}
+
+	conversion, ok := document.Paths["/api/v1/card-to-iban"]["post"]
+	if !ok {
+		t.Fatal("OpenAPI document does not define POST /api/v1/card-to-iban")
+	}
+	for _, status := range []string{"200", "400", "405", "422", "500", "502", "503", "504"} {
+		if _, ok := conversion.Responses[status]; !ok {
+			t.Errorf("POST /api/v1/card-to-iban does not document response %s", status)
+		}
+	}
+	for _, schema := range []string{
+		"ConvertRequest",
+		"ConvertResponse",
+		"ErrorResponse",
+		"HealthResponse",
+	} {
+		if _, ok := document.Components.Schemas[schema]; !ok {
+			t.Errorf("OpenAPI document does not define schema %s", schema)
+		}
+	}
+}

backend/internal/httpapi/handler.go

@@ -60,6 +60,7 @@ func NewHandler(
 func (h *Handler) Convert(w http.ResponseWriter, r *http.Request) {
 	startedAt := h.now()
 	requestID := h.newRequestID()
+	clientIP := extractClientIP(r)
 
 	w.Header().Set("X-Request-ID", requestID)
 	w.Header().Set("X-Content-Type-Options", "nosniff")
@@ -89,6 +90,7 @@ func (h *Handler) Convert(w http.ResponseWriter, r *http.Request) {
 		RequestID:  requestID,
 		CreatedAt:  startedAt,
 		Duration:   duration,
+		ClientIP:   clientIP,
 		Status:     outcome.auditStatus,
 		MaskedCard: card.Mask(cardNumber),
 		ResponseJSON: audit.SanitizeResponse(
@@ -102,11 +104,17 @@ func (h *Handler) Convert(w http.ResponseWriter, r *http.Request) {
 		record.UpstreamHTTPStatus = &upstreamStatus
 	}
 
+	var clientIPValue string
+	if clientIP != nil {
+		clientIPValue = *clientIP
+	}
+
 	if err := h.audits.Save(r.Context(), record); err != nil {
 		h.logger.ErrorContext(
 			r.Context(),
 			"request audit failed",
 			"request_id", requestID,
+			"client_ip", clientIPValue,
 			"route", "/api/v1/card-to-iban",
 			"http_status", http.StatusInternalServerError,
 			"audit_status", audit.StatusInternalError,
@@ -128,6 +136,7 @@ func (h *Handler) Convert(w http.ResponseWriter, r *http.Request) {
 		r.Context(),
 		"request completed",
 		"request_id", requestID,
+		"client_ip", clientIPValue,
 		"route", "/api/v1/card-to-iban",
 		"http_status", outcome.httpStatus,
 		"audit_status", outcome.auditStatus,

backend/internal/httpapi/handler_test.go

@@ -248,6 +248,10 @@ func TestHandlerConvert(t *testing.T) {
 				t.Errorf("masked card = %q, want %q", record.MaskedCard, tt.wantMaskedCard)
 			}
 
+			if record.ClientIP == nil || *record.ClientIP != "192.0.2.1" {
+				t.Errorf("client IP = %v, want 192.0.2.1", record.ClientIP)
+			}
+
 			if strings.Contains(string(record.ResponseJSON), "6037997512345678") {
 				t.Error("audit response contains the full card number")
 			}
@@ -291,3 +295,75 @@ func TestHandlerFailsClosedWhenAuditSaveFails(t *testing.T) {
 		t.Errorf("response body = %s, want internal_error", response.Body.String())
 	}
 }
+
+func TestHandlerExtractsClientIP(t *testing.T) {
+	tests := []struct {
+		name       string
+		headerKey  string
+		headerVal  string
+		remoteAddr string
+		wantIP     string
+	}{
+		{
+			name:       "prefers CF-Connecting-IP",
+			headerKey:  "CF-Connecting-IP",
+			headerVal:  "203.0.113.88",
+			remoteAddr: "10.0.0.1:1234",
+			wantIP:     "203.0.113.88",
+		},
+		{
+			name:       "prefers X-Forwarded-For over RemoteAddr",
+			headerKey:  "X-Forwarded-For",
+			headerVal:  "198.51.100.77, 10.0.0.2",
+			remoteAddr: "10.0.0.1:1234",
+			wantIP:     "198.51.100.77",
+		},
+		{
+			name:       "uses RemoteAddr host without port",
+			remoteAddr: "192.168.10.5:9999",
+			wantIP:     "192.168.10.5",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			audits := &fakeAuditSaver{}
+			handler := NewHandler(
+				&fakeConverter{
+					result: service.ConversionResult{
+						IBAN:               "IR820540102680020817909002",
+						UpstreamHTTPStatus: http.StatusOK,
+					},
+				},
+				audits,
+				slog.New(slog.NewTextHandler(io.Discard, nil)),
+			)
+
+			req := httptest.NewRequest(
+				http.MethodPost,
+				"/api/v1/card-to-iban",
+				strings.NewReader(`{"card":"6037997512345678"}`),
+			)
+			if tt.headerKey != "" {
+				req.Header.Set(tt.headerKey, tt.headerVal)
+			}
+			req.RemoteAddr = tt.remoteAddr
+
+			rec := httptest.NewRecorder()
+			handler.Convert(rec, req)
+
+			if rec.Code != http.StatusOK {
+				t.Fatalf("status = %d, want 200", rec.Code)
+			}
+
+			if len(audits.records) != 1 {
+				t.Fatalf("audit records count = %d, want 1", len(audits.records))
+			}
+
+			recIP := audits.records[0].ClientIP
+			if recIP == nil || *recIP != tt.wantIP {
+				t.Errorf("ClientIP = %v, want %q", recIP, tt.wantIP)
+			}
+		})
+	}
+}

backend/internal/httpapi/openapi.json

@@ -0,0 +1,363 @@
+{
+  "openapi": "3.0.3",
+  "info": {
+    "title": "Card to IBAN API",
+    "version": "1.0.0",
+    "description": "Converts an Iranian bank card number to its IBAN."
+  },
+  "servers": [
+    {
+      "url": "/",
+      "description": "Current server"
+    }
+  ],
+  "tags": [
+    {
+      "name": "System"
+    },
+    {
+      "name": "Conversion"
+    }
+  ],
+  "paths": {
+    "/health": {
+      "get": {
+        "tags": ["System"],
+        "summary": "Check service health",
+        "operationId": "getHealth",
+        "responses": {
+          "200": {
+            "description": "The service is running.",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/HealthResponse"
+                },
+                "example": {
+                  "status": "ok"
+                }
+              }
+            }
+          }
+        }
+      }
+    },
+    "/api/v1/card-to-iban": {
+      "post": {
+        "tags": ["Conversion"],
+        "summary": "Convert a card number to an IBAN",
+        "operationId": "convertCardToIBAN",
+        "requestBody": {
+          "required": true,
+          "content": {
+            "application/json": {
+              "schema": {
+                "$ref": "#/components/schemas/ConvertRequest"
+              },
+              "example": {
+                "card": "6037997512345678"
+              }
+            }
+          }
+        },
+        "responses": {
+          "200": {
+            "description": "The card number was converted successfully.",
+            "headers": {
+              "X-Request-ID": {
+                "$ref": "#/components/headers/RequestID"
+              }
+            },
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ConvertResponse"
+                },
+                "example": {
+                  "data": {
+                    "iban": "IR820540102680020817909002"
+                  },
+                  "requestId": "SERVER_GENERATED_REQUEST_ID"
+                }
+              }
+            }
+          },
+          "400": {
+            "$ref": "#/components/responses/InvalidCard"
+          },
+          "405": {
+            "$ref": "#/components/responses/MethodNotAllowed"
+          },
+          "422": {
+            "$ref": "#/components/responses/ConversionRejected"
+          },
+          "500": {
+            "$ref": "#/components/responses/InternalError"
+          },
+          "502": {
+            "$ref": "#/components/responses/UpstreamFailure"
+          },
+          "503": {
+            "$ref": "#/components/responses/UpstreamUnavailable"
+          },
+          "504": {
+            "$ref": "#/components/responses/UpstreamTimeout"
+          }
+        }
+      }
+    }
+  },
+  "components": {
+    "headers": {
+      "RequestID": {
+        "description": "Identifier generated for tracing this request.",
+        "schema": {
+          "type": "string"
+        },
+        "example": "SERVER_GENERATED_REQUEST_ID"
+      }
+    },
+    "schemas": {
+      "HealthResponse": {
+        "type": "object",
+        "required": ["status"],
+        "properties": {
+          "status": {
+            "type": "string",
+            "enum": ["ok"]
+          }
+        }
+      },
+      "ConvertRequest": {
+        "type": "object",
+        "additionalProperties": false,
+        "required": ["card"],
+        "properties": {
+          "card": {
+            "type": "string",
+            "description": "Iranian bank card number containing exactly 16 ASCII digits.",
+            "pattern": "^[0-9]{16}$",
+            "minLength": 16,
+            "maxLength": 16,
+            "example": "6037997512345678"
+          }
+        }
+      },
+      "ConvertResponse": {
+        "type": "object",
+        "required": ["data", "requestId"],
+        "properties": {
+          "data": {
+            "$ref": "#/components/schemas/ConvertData"
+          },
+          "requestId": {
+            "type": "string",
+            "example": "SERVER_GENERATED_REQUEST_ID"
+          }
+        }
+      },
+      "ConvertData": {
+        "type": "object",
+        "required": ["iban"],
+        "properties": {
+          "iban": {
+            "type": "string",
+            "pattern": "^IR[0-9]{24}$",
+            "example": "IR820540102680020817909002"
+          }
+        }
+      },
+      "ErrorResponse": {
+        "type": "object",
+        "required": ["error", "requestId"],
+        "properties": {
+          "error": {
+            "$ref": "#/components/schemas/ErrorDetail"
+          },
+          "requestId": {
+            "type": "string",
+            "example": "SERVER_GENERATED_REQUEST_ID"
+          }
+        }
+      },
+      "ErrorDetail": {
+        "type": "object",
+        "required": ["code", "message"],
+        "properties": {
+          "code": {
+            "type": "string",
+            "enum": [
+              "invalid_card",
+              "method_not_allowed",
+              "conversion_rejected",
+              "internal_error",
+              "upstream_failure",
+              "upstream_unavailable",
+              "upstream_timeout"
+            ]
+          },
+          "message": {
+            "type": "string",
+            "description": "A user-facing error message in Persian."
+          }
+        }
+      }
+    },
+    "responses": {
+      "InvalidCard": {
+        "description": "The JSON body or card number is invalid.",
+        "headers": {
+          "X-Request-ID": {
+            "$ref": "#/components/headers/RequestID"
+          }
+        },
+        "content": {
+          "application/json": {
+            "schema": {
+              "$ref": "#/components/schemas/ErrorResponse"
+            },
+            "example": {
+              "error": {
+                "code": "invalid_card",
+                "message": "شماره کارت باید شامل ۱۶ رقم باشد."
+              },
+              "requestId": "SERVER_GENERATED_REQUEST_ID"
+            }
+          }
+        }
+      },
+      "MethodNotAllowed": {
+        "description": "The request method is not supported for this path.",
+        "headers": {
+          "X-Request-ID": {
+            "$ref": "#/components/headers/RequestID"
+          }
+        },
+        "content": {
+          "application/json": {
+            "schema": {
+              "$ref": "#/components/schemas/ErrorResponse"
+            },
+            "example": {
+              "error": {
+                "code": "method_not_allowed",
+                "message": "این روش برای این مسیر پشتیبانی نمی‌شود."
+              },
+              "requestId": "SERVER_GENERATED_REQUEST_ID"
+            }
+          }
+        }
+      },
+      "ConversionRejected": {
+        "description": "The upstream service rejected the conversion.",
+        "headers": {
+          "X-Request-ID": {
+            "$ref": "#/components/headers/RequestID"
+          }
+        },
+        "content": {
+          "application/json": {
+            "schema": {
+              "$ref": "#/components/schemas/ErrorResponse"
+            },
+            "example": {
+              "error": {
+                "code": "conversion_rejected",
+                "message": "تبدیل این شماره کارت انجام نشد."
+              },
+              "requestId": "SERVER_GENERATED_REQUEST_ID"
+            }
+          }
+        }
+      },
+      "InternalError": {
+        "description": "An internal operation failed.",
+        "headers": {
+          "X-Request-ID": {
+            "$ref": "#/components/headers/RequestID"
+          }
+        },
+        "content": {
+          "application/json": {
+            "schema": {
+              "$ref": "#/components/schemas/ErrorResponse"
+            },
+            "example": {
+              "error": {
+                "code": "internal_error",
+                "message": "خطایی در پردازش درخواست رخ داد."
+              },
+              "requestId": "SERVER_GENERATED_REQUEST_ID"
+            }
+          }
+        }
+      },
+      "UpstreamFailure": {
+        "description": "Upstream authentication failed or its response was invalid.",
+        "headers": {
+          "X-Request-ID": {
+            "$ref": "#/components/headers/RequestID"
+          }
+        },
+        "content": {
+          "application/json": {
+            "schema": {
+              "$ref": "#/components/schemas/ErrorResponse"
+            },
+            "example": {
+              "error": {
+                "code": "upstream_failure",
+                "message": "ارتباط با سرویس بانکی با خطا روبه‌رو شد."
+              },
+              "requestId": "SERVER_GENERATED_REQUEST_ID"
+            }
+          }
+        }
+      },
+      "UpstreamUnavailable": {
+        "description": "The upstream service is temporarily unavailable.",
+        "headers": {
+          "X-Request-ID": {
+            "$ref": "#/components/headers/RequestID"
+          }
+        },
+        "content": {
+          "application/json": {
+            "schema": {
+              "$ref": "#/components/schemas/ErrorResponse"
+            },
+            "example": {
+              "error": {
+                "code": "upstream_unavailable",
+                "message": "سرویس بانکی موقتاً در دسترس نیست."
+              },
+              "requestId": "SERVER_GENERATED_REQUEST_ID"
+            }
+          }
+        }
+      },
+      "UpstreamTimeout": {
+        "description": "The upstream request timed out.",
+        "headers": {
+          "X-Request-ID": {
+            "$ref": "#/components/headers/RequestID"
+          }
+        },
+        "content": {
+          "application/json": {
+            "schema": {
+              "$ref": "#/components/schemas/ErrorResponse"
+            },
+            "example": {
+              "error": {
+                "code": "upstream_timeout",
+                "message": "پاسخ سرویس بانکی بیش از حد طول کشید."
+              },
+              "requestId": "SERVER_GENERATED_REQUEST_ID"
+            }
+          }
+        }
+      }
+    }
+  }
+}

backend/internal/zarinhub/client.go

@@ -7,18 +7,28 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"log/slog"
 	"net"
 	"net/http"
+	"time"
 
 	"card-to-iban/internal/service"
 )
 
-const maxResponseBodySize = 1 << 20
+const (
+	maxResponseBodySize = 1 << 20
+
+	retryBaseDelay = 200 * time.Millisecond
+	retryMaxDelay  = 2 * time.Second
+)
 
 type Client struct {
 	httpClient    *http.Client
 	endpointURL   string
 	tokenProvider TokenProvider
+	maxRetries    int
+	retryWait     func(ctx context.Context, delay time.Duration) bool
+	logger        *slog.Logger
 }
 
 var _ service.CardToIBANClient = (*Client)(nil)
@@ -32,17 +42,114 @@ func NewClient(
 	httpClient *http.Client,
 	endpointURL string,
 	tokenProvider TokenProvider,
+	maxRetries int,
+	loggers ...*slog.Logger,
 ) *Client {
-	return &Client{
+	if maxRetries < 0 {
+		maxRetries = 0
+	}
+
+	var logger *slog.Logger
+	if len(loggers) > 0 && loggers[0] != nil {
+		logger = loggers[0]
+	}
+
+	if httpClient == nil {
+		httpClient = http.DefaultClient
+	}
+
+	client := &Client{
 		httpClient:    httpClient,
 		endpointURL:   endpointURL,
 		tokenProvider: tokenProvider,
+		maxRetries:    maxRetries,
+		logger:        logger,
+	}
+
+	if _, ok := httpClient.Transport.(*RetryTransport); !ok && maxRetries > 0 {
+		rt := NewRetryTransport(
+			httpClient.Transport,
+			httpClient.Timeout,
+			maxRetries,
+			logger,
+		)
+		rt.retryWait = func(ctx context.Context, delay time.Duration) bool {
+			if client.retryWait != nil {
+				return client.retryWait(ctx, delay)
+			}
+			return waitForRetry(ctx, delay)
+		}
+		client.httpClient = &http.Client{
+			Transport: rt,
+		}
+	} else if rt, ok := httpClient.Transport.(*RetryTransport); ok {
+		origWait := rt.retryWait
+		rt.retryWait = func(ctx context.Context, delay time.Duration) bool {
+			if client.retryWait != nil {
+				return client.retryWait(ctx, delay)
+			}
+			if origWait != nil {
+				return origWait(ctx, delay)
+			}
+			return waitForRetry(ctx, delay)
+		}
+	}
+
+	return client
+}
+
+func (c *Client) log() *slog.Logger {
+	if c.logger != nil {
+		return c.logger
+	}
+	return slog.New(slog.NewTextHandler(io.Discard, nil))
+}
+
+func isRetryable(err error) bool {
+	return errors.Is(err, ErrTimeout) ||
+		errors.Is(err, ErrUnavailable)
+}
+
+func retryDelay(attempt int) time.Duration {
+	delay := retryBaseDelay
+
+	for i := 0; i < attempt; i++ {
+		delay *= 2
+
+		if delay >= retryMaxDelay {
+			return retryMaxDelay
+		}
+	}
+
+	return delay
+}
+
+func waitForRetry(
+	ctx context.Context,
+	delay time.Duration,
+) bool {
+	timer := time.NewTimer(delay)
+	defer timer.Stop()
+
+	select {
+	case <-ctx.Done():
+		return false
+
+	case <-timer.C:
+		return true
 	}
 }
 
 func (c *Client) Convert(
 	ctx context.Context,
 	cardNumber string,
+) (service.ConversionResult, error) {
+	return c.convertOnce(ctx, cardNumber)
+}
+
+func (c *Client) convertOnce(
+	ctx context.Context,
+	cardNumber string,
 ) (service.ConversionResult, error) {
 	accessToken, err := c.tokenProvider.AccessToken(ctx)
 	if err != nil {

backend/internal/zarinhub/client_test.go

@@ -1,12 +1,15 @@
 package zarinhub
 
 import (
+	"bytes"
 	"context"
 	"encoding/json"
 	"errors"
 	"fmt"
+	"log/slog"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
 	"time"
 )
@@ -98,6 +101,7 @@ func TestClientConvertSuccess(t *testing.T) {
 		},
 		server.URL,
 		&fakeTokenProvider{tokens: []string{token}},
+		0,
 	)
 
 	result, err := client.Convert(
@@ -172,6 +176,7 @@ func TestClientReturnsRejectedForApplicationFailure(
 		},
 		server.URL,
 		&fakeTokenProvider{tokens: []string{"test-token"}},
+		0,
 	)
 
 	result, err := client.Convert(
@@ -264,6 +269,7 @@ func TestClientMapsHTTPStatusErrors(t *testing.T) {
 					},
 					server.URL,
 					&fakeTokenProvider{tokens: []string{"test-token"}},
+					0,
 				)
 
 				result, err := client.Convert(
@@ -322,6 +328,7 @@ func TestClientReturnsInvalidResponseForMalformedJSON(
 		},
 		server.URL,
 		&fakeTokenProvider{tokens: []string{"test-token"}},
+		0,
 	)
 
 	result, err := client.Convert(
@@ -367,6 +374,7 @@ func TestClientReturnsTimeout(t *testing.T) {
 		},
 		server.URL,
 		&fakeTokenProvider{tokens: []string{"test-token"}},
+		0,
 	)
 
 	_, err := client.Convert(
@@ -403,6 +411,7 @@ func TestClientRejectsOversizedResponse(t *testing.T) {
 		&http.Client{Timeout: time.Second},
 		server.URL,
 		&fakeTokenProvider{tokens: []string{"test-token"}},
+		0,
 	)
 
 	_, err := client.Convert(
@@ -442,7 +451,7 @@ func TestClientRefreshesAndRetriesUnauthorizedRequest(t *testing.T) {
 	defer server.Close()
 
 	tokens := &fakeTokenProvider{tokens: []string{"expired-token", "fresh-token"}}
-	client := NewClient(server.Client(), server.URL, tokens)
+	client := NewClient(server.Client(), server.URL, tokens, 0)
 
 	result, err := client.Convert(context.Background(), "6037997512345678")
 	if err != nil {
@@ -478,3 +487,280 @@ func (p *fakeTokenProvider) AccessToken(context.Context) (string, error) {
 func (p *fakeTokenProvider) Invalidate(token string) {
 	p.invalidated = append(p.invalidated, token)
 }
+
+func TestRetryDelay(t *testing.T) {
+	tests := []struct {
+		attempt int
+		want    time.Duration
+	}{
+		{attempt: -1, want: 200 * time.Millisecond},
+		{attempt: 0, want: 200 * time.Millisecond},
+		{attempt: 1, want: 400 * time.Millisecond},
+		{attempt: 2, want: 800 * time.Millisecond},
+		{attempt: 3, want: 1600 * time.Millisecond},
+		{attempt: 4, want: 2 * time.Second},
+		{attempt: 10, want: 2 * time.Second},
+	}
+
+	for _, tt := range tests {
+		got := retryDelay(tt.attempt)
+		if got != tt.want {
+			t.Errorf("retryDelay(%d) = %v, want %v", tt.attempt, got, tt.want)
+		}
+	}
+}
+
+func TestNewClientSanitizesNegativeRetries(t *testing.T) {
+	client := NewClient(nil, "", nil, -1)
+	if client.maxRetries != 0 {
+		t.Errorf("maxRetries = %d, want 0", client.maxRetries)
+	}
+}
+
+func TestClientRetriesOnTransientErrorsUntilSuccess(t *testing.T) {
+	const expectedIBAN = "IR820540102680020817909002"
+	requestCount := 0
+
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		requestCount++
+		if requestCount == 1 {
+			w.WriteHeader(http.StatusServiceUnavailable)
+			fmt.Fprint(w, `{"error":"temporary outage"}`)
+			return
+		}
+
+		w.Header().Set("Content-Type", "application/json")
+		fmt.Fprintf(w, `{
+			"data":{"iban":%q},
+			"meta":{"isSuccess":true,"errors":[]}
+		}`, expectedIBAN)
+	}))
+	defer server.Close()
+
+	var recordedDelays []time.Duration
+	client := NewClient(
+		&http.Client{Timeout: time.Second},
+		server.URL,
+		&fakeTokenProvider{tokens: []string{"test-token"}},
+		2,
+	)
+	client.retryWait = func(ctx context.Context, delay time.Duration) bool {
+		recordedDelays = append(recordedDelays, delay)
+		return true
+	}
+
+	result, err := client.Convert(context.Background(), "6037997512345678")
+	if err != nil {
+		t.Fatalf("Convert() unexpected error = %v", err)
+	}
+	if result.IBAN != expectedIBAN {
+		t.Errorf("IBAN = %q, want %q", result.IBAN, expectedIBAN)
+	}
+	if requestCount != 2 {
+		t.Errorf("request count = %d, want 2", requestCount)
+	}
+	if len(recordedDelays) != 1 {
+		t.Fatalf("len(recordedDelays) = %d, want 1", len(recordedDelays))
+	}
+	if recordedDelays[0] != 200*time.Millisecond {
+		t.Errorf("delay[0] = %v, want 200ms", recordedDelays[0])
+	}
+}
+
+func TestClientRetriesExhausted(t *testing.T) {
+	requestCount := 0
+
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		requestCount++
+		w.WriteHeader(http.StatusGatewayTimeout)
+		fmt.Fprint(w, `{"error":"gateway timeout"}`)
+	}))
+	defer server.Close()
+
+	var recordedDelays []time.Duration
+	client := NewClient(
+		&http.Client{Timeout: time.Second},
+		server.URL,
+		&fakeTokenProvider{tokens: []string{"test-token"}},
+		2,
+	)
+	client.retryWait = func(ctx context.Context, delay time.Duration) bool {
+		recordedDelays = append(recordedDelays, delay)
+		return true
+	}
+
+	_, err := client.Convert(context.Background(), "6037997512345678")
+	if !errors.Is(err, ErrUnavailable) {
+		t.Fatalf("error = %v, want ErrUnavailable", err)
+	}
+	if requestCount != 3 {
+		t.Errorf("request count = %d, want 3", requestCount)
+	}
+	if len(recordedDelays) != 2 {
+		t.Fatalf("len(recordedDelays) = %d, want 2", len(recordedDelays))
+	}
+	if recordedDelays[0] != 200*time.Millisecond {
+		t.Errorf("delay[0] = %v, want 200ms", recordedDelays[0])
+	}
+	if recordedDelays[1] != 400*time.Millisecond {
+		t.Errorf("delay[1] = %v, want 400ms", recordedDelays[1])
+	}
+}
+
+func TestClientDoesNotRetryNonTransientErrors(t *testing.T) {
+	requestCount := 0
+
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		requestCount++
+		w.WriteHeader(http.StatusBadRequest)
+		fmt.Fprint(w, `{"error":"invalid card format"}`)
+	}))
+	defer server.Close()
+
+	client := NewClient(
+		&http.Client{Timeout: time.Second},
+		server.URL,
+		&fakeTokenProvider{tokens: []string{"test-token"}},
+		2,
+	)
+	client.retryWait = func(ctx context.Context, delay time.Duration) bool {
+		t.Error("retryWait should not be called for non-transient errors")
+		return true
+	}
+
+	_, err := client.Convert(context.Background(), "6037997512345678")
+	if !errors.Is(err, ErrRejected) {
+		t.Fatalf("error = %v, want ErrRejected", err)
+	}
+	if requestCount != 1 {
+		t.Errorf("request count = %d, want 1", requestCount)
+	}
+}
+
+func TestClientStopsRetryOnContextCancellation(t *testing.T) {
+	requestCount := 0
+
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		requestCount++
+		w.WriteHeader(http.StatusServiceUnavailable)
+	}))
+	defer server.Close()
+
+	ctx, cancel := context.WithCancel(context.Background())
+
+	client := NewClient(
+		&http.Client{Timeout: time.Second},
+		server.URL,
+		&fakeTokenProvider{tokens: []string{"test-token"}},
+		2,
+	)
+	client.retryWait = func(c context.Context, delay time.Duration) bool {
+		cancel()
+		return waitForRetry(ctx, delay)
+	}
+
+	_, err := client.Convert(ctx, "6037997512345678")
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+	if !errors.Is(err, context.Canceled) {
+		t.Errorf("error = %v, want context.Canceled", err)
+	}
+	if requestCount != 1 {
+		t.Errorf("request count = %d, want 1", requestCount)
+	}
+}
+
+func TestClientRetryLogging(t *testing.T) {
+	const expectedIBAN = "IR820540102680020817909002"
+	requestCount := 0
+
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		requestCount++
+		if requestCount == 1 {
+			w.WriteHeader(http.StatusServiceUnavailable)
+			fmt.Fprint(w, `{"error":"temporary outage"}`)
+			return
+		}
+
+		w.Header().Set("Content-Type", "application/json")
+		fmt.Fprintf(w, `{
+			"data":{"iban":%q},
+			"meta":{"isSuccess":true,"errors":[]}
+		}`, expectedIBAN)
+	}))
+	defer server.Close()
+
+	var logBuf bytes.Buffer
+	logger := slog.New(slog.NewJSONHandler(&logBuf, nil))
+
+	client := NewClient(
+		&http.Client{Timeout: time.Second},
+		server.URL,
+		&fakeTokenProvider{tokens: []string{"test-token"}},
+		2,
+		logger,
+	)
+	client.retryWait = func(ctx context.Context, delay time.Duration) bool {
+		return true
+	}
+
+	result, err := client.Convert(context.Background(), "6037997512345678")
+	if err != nil {
+		t.Fatalf("Convert() unexpected error = %v", err)
+	}
+	if result.IBAN != expectedIBAN {
+		t.Errorf("IBAN = %q, want %q", result.IBAN, expectedIBAN)
+	}
+
+	logs := logBuf.String()
+
+	if !strings.Contains(logs, "[INITIAL] zarinhub request") {
+		t.Errorf("expected logs to contain '[INITIAL] zarinhub request', got:\n%s", logs)
+	}
+	if !strings.Contains(logs, `"is_retry":false`) {
+		t.Errorf("expected logs to contain '\"is_retry\":false', got:\n%s", logs)
+	}
+
+	if !strings.Contains(logs, "[RETRY] zarinhub request attempt 1") {
+		t.Errorf("expected logs to contain '[RETRY] zarinhub request attempt 1', got:\n%s", logs)
+	}
+	if !strings.Contains(logs, "[RETRY] zarinhub request failed, retrying in") {
+		t.Errorf("expected logs to contain '[RETRY] zarinhub request failed, retrying in', got:\n%s", logs)
+	}
+	if !strings.Contains(logs, `"is_retry":true`) {
+		t.Errorf("expected logs to contain '\"is_retry\":true', got:\n%s", logs)
+	}
+}
+
+func TestClientRetryLoggingExhausted(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusServiceUnavailable)
+		fmt.Fprint(w, `{"error":"down"}`)
+	}))
+	defer server.Close()
+
+	var logBuf bytes.Buffer
+	logger := slog.New(slog.NewJSONHandler(&logBuf, nil))
+
+	client := NewClient(
+		&http.Client{Timeout: time.Second},
+		server.URL,
+		&fakeTokenProvider{tokens: []string{"test-token"}},
+		1,
+		logger,
+	)
+	client.retryWait = func(ctx context.Context, delay time.Duration) bool {
+		return true
+	}
+
+	_, err := client.Convert(context.Background(), "6037997512345678")
+	if !errors.Is(err, ErrUnavailable) {
+		t.Fatalf("error = %v, want ErrUnavailable", err)
+	}
+
+	logs := logBuf.String()
+	if !strings.Contains(logs, "[RETRY] zarinhub request failed: all retries exhausted") {
+		t.Errorf("expected logs to contain exhausted retries message, got:\n%s", logs)
+	}
+}

backend/internal/zarinhub/token_manager_test.go

@@ -3,6 +3,7 @@ package zarinhub
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"net/http"
 	"net/http/httptest"
 	"sync"
@@ -220,3 +221,120 @@ func writeTokenResponse(
 		t.Fatalf("encode response: %v", err)
 	}
 }
+
+func TestTokenManagerRetriesOnTransientErrorsUntilSuccess(t *testing.T) {
+	var calls int
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		calls++
+		if calls == 1 {
+			w.WriteHeader(http.StatusServiceUnavailable)
+			return
+		}
+		writeTokenResponse(t, w, "retried-access", "retried-refresh", 28800)
+	}))
+	defer server.Close()
+
+	var recordedDelays []time.Duration
+	transport := NewRetryTransport(
+		server.Client().Transport,
+		time.Second,
+		2,
+	)
+	transport.retryWait = func(ctx context.Context, delay time.Duration) bool {
+		recordedDelays = append(recordedDelays, delay)
+		return true
+	}
+	httpClient := &http.Client{Transport: transport}
+
+	manager := NewTokenManager(httpClient, server.URL, "test-user", "test-pass")
+	token, err := manager.AccessToken(context.Background())
+	if err != nil {
+		t.Fatalf("AccessToken() error = %v", err)
+	}
+	if token != "retried-access" {
+		t.Errorf("token = %q, want retried-access", token)
+	}
+	if calls != 2 {
+		t.Errorf("calls = %d, want 2", calls)
+	}
+	if len(recordedDelays) != 1 {
+		t.Fatalf("len(recordedDelays) = %d, want 1", len(recordedDelays))
+	}
+	if recordedDelays[0] != 200*time.Millisecond {
+		t.Errorf("delay[0] = %v, want 200ms", recordedDelays[0])
+	}
+}
+
+func TestTokenManagerRetriesOnTimeoutUntilSuccess(t *testing.T) {
+	var calls int
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		calls++
+		if calls == 1 {
+			time.Sleep(150 * time.Millisecond)
+			w.WriteHeader(http.StatusOK)
+			return
+		}
+		writeTokenResponse(t, w, "timeout-recovered-access", "timeout-recovered-refresh", 28800)
+	}))
+	defer server.Close()
+
+	var recordedDelays []time.Duration
+	transport := NewRetryTransport(
+		server.Client().Transport,
+		50*time.Millisecond,
+		2,
+	)
+	transport.retryWait = func(ctx context.Context, delay time.Duration) bool {
+		recordedDelays = append(recordedDelays, delay)
+		return true
+	}
+	httpClient := &http.Client{Transport: transport}
+
+	manager := NewTokenManager(httpClient, server.URL, "test-user", "test-pass")
+	token, err := manager.AccessToken(context.Background())
+	if err != nil {
+		t.Fatalf("AccessToken() error = %v", err)
+	}
+	if token != "timeout-recovered-access" {
+		t.Errorf("token = %q, want timeout-recovered-access", token)
+	}
+	if calls != 2 {
+		t.Errorf("calls = %d, want 2", calls)
+	}
+	if len(recordedDelays) != 1 {
+		t.Fatalf("len(recordedDelays) = %d, want 1", len(recordedDelays))
+	}
+}
+
+func TestTokenManagerRetriesExhausted(t *testing.T) {
+	var calls int
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		calls++
+		w.WriteHeader(http.StatusBadGateway)
+	}))
+	defer server.Close()
+
+	var recordedDelays []time.Duration
+	transport := NewRetryTransport(
+		server.Client().Transport,
+		time.Second,
+		2,
+	)
+	transport.retryWait = func(ctx context.Context, delay time.Duration) bool {
+		recordedDelays = append(recordedDelays, delay)
+		return true
+	}
+	httpClient := &http.Client{Transport: transport}
+
+	manager := NewTokenManager(httpClient, server.URL, "test-user", "test-pass")
+	_, err := manager.AccessToken(context.Background())
+	if !errors.Is(err, ErrUnavailable) {
+		t.Fatalf("error = %v, want ErrUnavailable", err)
+	}
+	if calls != 3 {
+		t.Errorf("calls = %d, want 3", calls)
+	}
+	if len(recordedDelays) != 2 {
+		t.Fatalf("len(recordedDelays) = %d, want 2", len(recordedDelays))
+	}
+}

backend/internal/zarinhub/transport.go

@@ -0,0 +1,239 @@
+package zarinhub
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"log/slog"
+	"net/http"
+	"time"
+)
+
+type RetryTransport struct {
+	base       http.RoundTripper
+	timeout    time.Duration
+	maxRetries int
+	retryWait  func(ctx context.Context, delay time.Duration) bool
+	logger     *slog.Logger
+}
+
+func NewRetryTransport(
+	base http.RoundTripper,
+	timeout time.Duration,
+	maxRetries int,
+	loggers ...*slog.Logger,
+) *RetryTransport {
+	if base == nil {
+		base = http.DefaultTransport
+	}
+	if maxRetries < 0 {
+		maxRetries = 0
+	}
+
+	var logger *slog.Logger
+	if len(loggers) > 0 && loggers[0] != nil {
+		logger = loggers[0]
+	}
+
+	return &RetryTransport{
+		base:       base,
+		timeout:    timeout,
+		maxRetries: maxRetries,
+		retryWait:  waitForRetry,
+		logger:     logger,
+	}
+}
+
+func NewHTTPClient(
+	timeout time.Duration,
+	maxRetries int,
+	loggers ...*slog.Logger,
+) *http.Client {
+	transport := NewRetryTransport(
+		http.DefaultTransport,
+		timeout,
+		maxRetries,
+		loggers...,
+	)
+
+	return &http.Client{
+		Transport: transport,
+	}
+}
+
+func (t *RetryTransport) log() *slog.Logger {
+	if t.logger != nil {
+		return t.logger
+	}
+	return slog.New(slog.NewTextHandler(io.Discard, nil))
+}
+
+func (t *RetryTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+	waitFn := t.retryWait
+	if waitFn == nil {
+		waitFn = waitForRetry
+	}
+
+	if req.Body != nil && req.GetBody == nil {
+		bodyBytes, err := io.ReadAll(req.Body)
+		if err != nil {
+			return nil, err
+		}
+		_ = req.Body.Close()
+		req.GetBody = func() (io.ReadCloser, error) {
+			return io.NopCloser(bytes.NewReader(bodyBytes)), nil
+		}
+	}
+
+	var (
+		resp *http.Response
+		err  error
+	)
+
+	for attempt := 0; attempt <= t.maxRetries; attempt++ {
+		if attempt > 0 {
+			t.log().InfoContext(
+				req.Context(),
+				fmt.Sprintf("[RETRY] zarinhub request attempt %d", attempt),
+				"is_retry", true,
+				"retry_count", attempt,
+			)
+		} else {
+			t.log().InfoContext(
+				req.Context(),
+				"[INITIAL] zarinhub request",
+				"is_retry", false,
+				"retry_count", 0,
+			)
+		}
+
+		var (
+			attemptCtx    context.Context
+			cancelAttempt context.CancelFunc
+		)
+		if t.timeout > 0 {
+			attemptCtx, cancelAttempt = context.WithTimeout(req.Context(), t.timeout)
+		} else {
+			attemptCtx = req.Context()
+			cancelAttempt = func() {}
+		}
+
+		attemptReq := req.Clone(attemptCtx)
+		if req.GetBody != nil {
+			body, bodyErr := req.GetBody()
+			if bodyErr != nil {
+				cancelAttempt()
+				return nil, bodyErr
+			}
+			attemptReq.Body = body
+		}
+
+		resp, err = t.base.RoundTrip(attemptReq)
+
+		if req.Context().Err() != nil {
+			cancelAttempt()
+			if resp != nil && resp.Body != nil {
+				_ = resp.Body.Close()
+			}
+			if errors.Is(req.Context().Err(), context.Canceled) {
+				return nil, fmt.Errorf("zarinhub request canceled: %w", req.Context().Err())
+			}
+			return nil, req.Context().Err()
+		}
+
+		isErrRetryable := false
+		var retryReason error
+
+		if err != nil {
+			isErrRetryable = isTransportErrorRetryable(err)
+			retryReason = err
+		} else if isStatusCodeRetryable(resp.StatusCode) {
+			isErrRetryable = true
+			retryReason = fmt.Errorf("upstream returned status %d", resp.StatusCode)
+		}
+
+		if !isErrRetryable {
+			if resp != nil && resp.Body != nil {
+				resp.Body = &bodyWithCancel{
+					ReadCloser: resp.Body,
+					cancel:     cancelAttempt,
+				}
+			} else {
+				cancelAttempt()
+			}
+			return resp, err
+		}
+
+		if attempt == t.maxRetries {
+			if attempt > 0 {
+				t.log().ErrorContext(
+					req.Context(),
+					"[RETRY] zarinhub request failed: all retries exhausted",
+					"is_retry", true,
+					"retry_count", attempt,
+					"error", retryReason,
+				)
+			}
+			if resp != nil && resp.Body != nil {
+				resp.Body = &bodyWithCancel{
+					ReadCloser: resp.Body,
+					cancel:     cancelAttempt,
+				}
+			} else {
+				cancelAttempt()
+			}
+			return resp, err
+		}
+
+		if resp != nil && resp.Body != nil {
+			_, _ = io.Copy(io.Discard, io.LimitReader(resp.Body, 4096))
+			_ = resp.Body.Close()
+		}
+		cancelAttempt()
+
+		delay := retryDelay(attempt)
+		t.log().WarnContext(
+			req.Context(),
+			fmt.Sprintf("[RETRY] zarinhub request failed, retrying in %v (retry %d)", delay, attempt+1),
+			"is_retry", true,
+			"next_retry", attempt+1,
+			"delay_ms", delay.Milliseconds(),
+			"error", retryReason,
+		)
+
+		if !waitFn(req.Context(), delay) {
+			if req.Context().Err() != nil {
+				if errors.Is(req.Context().Err(), context.Canceled) {
+					return nil, fmt.Errorf("zarinhub request canceled: %w", req.Context().Err())
+				}
+				return nil, req.Context().Err()
+			}
+			return nil, context.Canceled
+		}
+	}
+
+	return resp, err
+}
+
+func isTransportErrorRetryable(err error) bool {
+	return !errors.Is(err, context.Canceled)
+}
+
+func isStatusCodeRetryable(statusCode int) bool {
+	return statusCode == http.StatusRequestTimeout ||
+		statusCode == http.StatusTooManyRequests ||
+		statusCode >= http.StatusInternalServerError
+}
+
+type bodyWithCancel struct {
+	io.ReadCloser
+	cancel context.CancelFunc
+}
+
+func (b *bodyWithCancel) Close() error {
+	err := b.ReadCloser.Close()
+	b.cancel()
+	return err
+}

backend/internal/zarinhub/transport_test.go

@@ -0,0 +1,211 @@
+package zarinhub
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func TestRetryTransportPreservesAndRewindsBody(t *testing.T) {
+	var attempts int32
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		att := atomic.AddInt32(&attempts, 1)
+		body, err := io.ReadAll(r.Body)
+		if err != nil {
+			t.Errorf("read body error: %v", err)
+		}
+		if string(body) != `{"payload":"important"}` {
+			t.Errorf("unexpected body on attempt %d: %q", att, string(body))
+		}
+		if att < 3 {
+			w.WriteHeader(http.StatusServiceUnavailable)
+			return
+		}
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte(`{"status":"ok"}`))
+	}))
+	defer server.Close()
+
+	transport := NewRetryTransport(server.Client().Transport, time.Second, 3)
+	transport.retryWait = func(ctx context.Context, delay time.Duration) bool {
+		return true
+	}
+	client := &http.Client{Transport: transport}
+
+	req, err := http.NewRequestWithContext(
+		context.Background(),
+		http.MethodPost,
+		server.URL,
+		bytes.NewReader([]byte(`{"payload":"important"}`)),
+	)
+	if err != nil {
+		t.Fatalf("NewRequest error: %v", err)
+	}
+
+	resp, err := client.Do(req)
+	if err != nil {
+		t.Fatalf("client.Do error: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		t.Errorf("status = %d, want %d", resp.StatusCode, http.StatusOK)
+	}
+	if attempts != 3 {
+		t.Errorf("attempts = %d, want 3", attempts)
+	}
+}
+
+func TestRetryTransportDoesNotRetryClientErrors(t *testing.T) {
+	tests := []int{
+		http.StatusBadRequest,
+		http.StatusUnauthorized,
+		http.StatusForbidden,
+		http.StatusNotFound,
+		http.StatusUnprocessableEntity,
+	}
+
+	for _, statusCode := range tests {
+		t.Run(fmt.Sprintf("status_%d", statusCode), func(t *testing.T) {
+			var attempts int32
+			server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				atomic.AddInt32(&attempts, 1)
+				w.WriteHeader(statusCode)
+			}))
+			defer server.Close()
+
+			transport := NewRetryTransport(server.Client().Transport, time.Second, 2)
+			client := &http.Client{Transport: transport}
+
+			req, _ := http.NewRequestWithContext(context.Background(), http.MethodGet, server.URL, nil)
+			resp, err := client.Do(req)
+			if err != nil {
+				t.Fatalf("client.Do error: %v", err)
+			}
+			defer resp.Body.Close()
+
+			if resp.StatusCode != statusCode {
+				t.Errorf("statusCode = %d, want %d", resp.StatusCode, statusCode)
+			}
+			if attempts != 1 {
+				t.Errorf("attempts = %d, want 1", attempts)
+			}
+		})
+	}
+}
+
+func TestRetryTransportPerAttemptTimeout(t *testing.T) {
+	var attempts int32
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		att := atomic.AddInt32(&attempts, 1)
+		if att == 1 {
+			time.Sleep(150 * time.Millisecond)
+			w.WriteHeader(http.StatusOK)
+			return
+		}
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer server.Close()
+
+	transport := NewRetryTransport(server.Client().Transport, 40*time.Millisecond, 2)
+	transport.retryWait = func(ctx context.Context, delay time.Duration) bool {
+		return true
+	}
+	client := &http.Client{Transport: transport}
+
+	// Parent context has plenty of time (5s)
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	req, _ := http.NewRequestWithContext(ctx, http.MethodGet, server.URL, nil)
+	resp, err := client.Do(req)
+	if err != nil {
+		t.Fatalf("client.Do error: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		t.Errorf("status = %d, want 200", resp.StatusCode)
+	}
+	if attempts != 2 {
+		t.Errorf("attempts = %d, want 2", attempts)
+	}
+}
+
+func TestRetryTransportParentContextCancellation(t *testing.T) {
+	var attempts int32
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		atomic.AddInt32(&attempts, 1)
+		w.WriteHeader(http.StatusServiceUnavailable)
+	}))
+	defer server.Close()
+
+	ctx, cancel := context.WithCancel(context.Background())
+	transport := NewRetryTransport(server.Client().Transport, time.Second, 3)
+	transport.retryWait = func(c context.Context, delay time.Duration) bool {
+		cancel() // cancel parent context during backoff
+		return waitForRetry(ctx, delay)
+	}
+	client := &http.Client{Transport: transport}
+
+	req, _ := http.NewRequestWithContext(ctx, http.MethodGet, server.URL, nil)
+	_, err := client.Do(req)
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+	if !errors.Is(err, context.Canceled) {
+		t.Errorf("error = %v, want context.Canceled", err)
+	}
+	if attempts != 1 {
+		t.Errorf("attempts = %d, want 1", attempts)
+	}
+}
+
+func TestRetryTransportLoggingFormat(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusServiceUnavailable)
+	}))
+	defer server.Close()
+
+	var buf bytes.Buffer
+	logger := slog.New(slog.NewJSONHandler(&buf, nil))
+
+	transport := NewRetryTransport(server.Client().Transport, time.Second, 1, logger)
+	transport.retryWait = func(ctx context.Context, delay time.Duration) bool {
+		return true
+	}
+	client := &http.Client{Transport: transport}
+
+	req, _ := http.NewRequestWithContext(context.Background(), http.MethodGet, server.URL, nil)
+	resp, err := client.Do(req)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	defer resp.Body.Close()
+
+	logStr := buf.String()
+	if !strings.Contains(logStr, "[INITIAL] zarinhub request") {
+		t.Errorf("expected logs to contain '[INITIAL] zarinhub request', got: %s", logStr)
+	}
+	if !strings.Contains(logStr, `"is_retry":false`) {
+		t.Errorf("expected logs to contain '\"is_retry\":false', got: %s", logStr)
+	}
+	if !strings.Contains(logStr, "[RETRY] zarinhub request attempt 1") {
+		t.Errorf("expected logs to contain '[RETRY] zarinhub request attempt 1', got: %s", logStr)
+	}
+	if !strings.Contains(logStr, "[RETRY] zarinhub request failed, retrying in") {
+		t.Errorf("expected logs to contain retry warning, got: %s", logStr)
+	}
+	if !strings.Contains(logStr, "[RETRY] zarinhub request failed: all retries exhausted") {
+		t.Errorf("expected logs to contain exhausted message, got: %s", logStr)
+	}
+}

frontend/app/app.vue

@@ -1,37 +1,3 @@
-<script setup lang="ts">
-const config = useRuntimeConfig()
-</script>
-
 <template>
-  <div class="min-h-dvh">
-    <header class="border-border bg-surface h-14 border-b shadow-[0_1px_2px_rgba(0,0,0,.03)]">
-      <div class="mx-auto flex h-full w-full max-w-[1120px] items-center px-4 sm:px-6 lg:px-8">
-        <BrandLogo />
-      </div>
-    </header>
-
-    <main class="mx-auto w-full max-w-[760px] px-4 py-10 sm:px-6 sm:py-14">
-      <section aria-labelledby="page-title">
-        <p class="text-brand mb-2 text-sm font-bold">سرویس تبدیل کارت به شبا</p>
-        <h1 id="page-title" class="text-foreground text-2xl leading-[1.5] font-extrabold sm:text-3xl">
-          شماره شبا را سریع و مطمئن پیدا کنید
-        </h1>
-        <p class="text-muted mt-3 max-w-[620px] text-sm leading-8 sm:text-[15px]">
-          استعلام مستقیماً از سرویس زرین‌هاب انجام می‌شود. شماره کامل کارت در گزارش‌ها یا پاسخ عمومی نگهداری نمی‌شود.
-        </p>
-      </section>
-
-      <div class="mt-7 sm:mt-8">
-        <CardConverter :api-base-url="config.public.apiBaseUrl" />
-      </div>
-
-      <p class="text-muted mt-5 flex items-start gap-2 text-xs leading-6">
-        <svg viewBox="0 0 24 24" class="mt-0.5 size-4 shrink-0" fill="none" stroke="currentColor" stroke-width="1.8" aria-hidden="true">
-          <path d="M12 22s8-4 8-10V5l-8-3-8 3v7c0 6 8 10 8 10Z" />
-          <path d="m9 12 2 2 4-4" />
-        </svg>
-        برای حفاظت از اطلاعات بانکی، فقط نسخه پوشیده‌شده شماره کارت در سوابق فنی ثبت می‌شود.
-      </p>
-    </main>
-  </div>
+  <NuxtPage />
 </template>

frontend/app/components/CardConverter.test.ts

@@ -1,12 +1,24 @@
-import { flushPromises, mount } from '@vue/test-utils'
+import { enableAutoUnmount, flushPromises, mount } from '@vue/test-utils'
 import { afterEach, describe, expect, it, vi } from 'vitest'
 
 import CardConverter from './CardConverter.vue'
 
+enableAutoUnmount(afterEach)
+
 afterEach(() => {
   vi.unstubAllGlobals()
 })
 
+function getAlert() {
+  const alert = document.body.querySelector<HTMLElement>('[role="alert"]')
+
+  if (!alert) {
+    throw new Error('Expected an alert to be rendered')
+  }
+
+  return alert
+}
+
 describe('CardConverter', () => {
   it('shows client validation without calling the API', async () => {
     const fetchMock = vi.fn()
@@ -19,7 +31,7 @@ describe('CardConverter', () => {
     await wrapper.get('form').trigger('submit')
 
     expect(fetchMock).not.toHaveBeenCalled()
-    expect(wrapper.get('[role="alert"]').text()).toContain('۱۶ رقم')
+    expect(getAlert().textContent).toContain('۱۶ رقم')
   })
 
   it('renders the returned iban after a successful request', async () => {
@@ -46,8 +58,6 @@ describe('CardConverter', () => {
 
     expect(wrapper.text()).toContain('IR82 0540 1026 8002 0817 9090 02')
     expect(document.activeElement?.getAttribute('data-result')).toBe('iban')
-
-    wrapper.unmount()
   })
 
   it('renders the safe API error message', async () => {
@@ -74,8 +84,6 @@ describe('CardConverter', () => {
     await wrapper.get('form').trigger('submit')
     await flushPromises()
 
-    expect(wrapper.get('[role="alert"]').text()).toContain(
-      'تبدیل این شماره کارت انجام نشد.',
-    )
+    expect(getAlert().textContent).toContain('تبدیل این شماره کارت انجام نشد.')
   })
 })

frontend/app/components/CardConverter.vue

@@ -252,7 +252,7 @@ onBeforeUnmount(dismissToast)
       v-if="iban"
       ref="resultElement"
       data-result="iban"
-      class="border-border bg-accent/55 border-t p-5 outline-none sm:p-6"
+      class="border-border bg-accent/55 border-t px-4 py-5 outline-none sm:p-6"
       tabindex="-1"
       aria-live="polite"
     >
@@ -270,7 +270,7 @@ onBeforeUnmount(dismissToast)
       </div>
 
       <output
-        class="border-brand/15 bg-surface block overflow-x-auto rounded-lg border px-4 py-5 text-center font-mono text-lg font-bold tracking-[0.04em] whitespace-nowrap sm:text-xl"
+        class="border-brand/15 bg-surface block overflow-x-auto rounded-lg border px-2.5 py-4 text-center font-mono text-xs font-bold tracking-normal whitespace-nowrap min-[390px]:text-sm sm:px-4 sm:py-5 sm:text-xl sm:tracking-[0.04em]"
         dir="ltr"
       >{{ formattedIBAN }}</output>
     </div>

frontend/app/error.test.ts

@@ -0,0 +1,21 @@
+import { mount } from '@vue/test-utils'
+import { describe, expect, it } from 'vitest'
+
+import ErrorPage from './error.vue'
+
+describe('ErrorPage', () => {
+  it('renders a branded not-found page with a way home', () => {
+    const wrapper = mount(ErrorPage, {
+      props: {
+        error: {
+          statusCode: 404,
+          statusMessage: 'Page not found',
+        },
+      },
+    })
+
+    expect(wrapper.get('h1').text()).toBe('صفحه پیدا نشد')
+    expect(wrapper.get('a').attributes('href')).toBe('/')
+    expect(wrapper.get('[role="img"]').attributes('aria-label')).toBe('زرین‌پال')
+  })
+})

frontend/app/error.vue

@@ -0,0 +1,55 @@
+<script setup lang="ts">
+import type { NuxtError } from '#app'
+import { computed } from 'vue'
+
+import BrandLogo from './components/BrandLogo.vue'
+
+const props = defineProps<{
+  error: NuxtError
+}>()
+
+const isNotFound = computed(() => props.error.statusCode === 404)
+</script>
+
+<template>
+  <div class="min-h-dvh">
+    <header class="border-border bg-surface h-14 border-b shadow-[0_1px_2px_rgba(0,0,0,.03)]">
+      <div class="mx-auto flex h-full w-full max-w-[1120px] items-center px-4 sm:px-6 lg:px-8">
+        <BrandLogo />
+      </div>
+    </header>
+
+    <main class="mx-auto flex w-full max-w-[760px] items-center px-4 py-12 sm:min-h-[calc(100dvh-3.5rem)] sm:px-6 sm:py-16">
+      <section
+        class="card border-border bg-surface w-full border p-6 sm:p-8"
+        aria-labelledby="error-title"
+      >
+        <div class="bg-accent text-brand mb-5 grid size-12 place-items-center rounded-lg" aria-hidden="true">
+          <svg viewBox="0 0 24 24" class="size-6" fill="none" stroke="currentColor" stroke-width="1.8">
+            <path d="M12 8v5m0 3.5v.01" stroke-linecap="round" />
+            <circle cx="12" cy="12" r="9" />
+          </svg>
+        </div>
+
+        <p class="text-brand mb-2 text-sm font-bold">خطای {{ error.statusCode }}</p>
+        <h1 id="error-title" class="text-foreground text-2xl leading-[1.5] font-extrabold sm:text-3xl">
+          {{ isNotFound ? 'صفحه پیدا نشد' : 'خطایی رخ داده است' }}
+        </h1>
+        <p class="text-muted mt-3 max-w-[560px] text-sm leading-8 sm:text-[15px]">
+          {{
+            isNotFound
+              ? 'نشانی واردشده معتبر نیست یا صفحه موردنظر جابه‌جا شده است.'
+              : 'در نمایش این صفحه مشکلی پیش آمد. لطفاً دوباره تلاش کنید.'
+          }}
+        </p>
+
+        <a
+          href="/"
+          class="bg-brand hover:bg-brand-hover text-on-brand mt-6 inline-flex min-h-11 items-center justify-center rounded-lg px-5 text-sm font-bold transition-colors"
+        >
+          بازگشت به صفحه اصلی
+        </a>
+      </section>
+    </main>
+  </div>
+</template>

frontend/app/pages/index.vue

@@ -0,0 +1,37 @@
+<script setup lang="ts">
+const config = useRuntimeConfig()
+</script>
+
+<template>
+  <div class="min-h-dvh">
+    <header class="border-border bg-surface h-14 border-b shadow-[0_1px_2px_rgba(0,0,0,.03)]">
+      <div class="mx-auto flex h-full w-full max-w-[1120px] items-center px-4 sm:px-6 lg:px-8">
+        <BrandLogo />
+      </div>
+    </header>
+
+    <main class="mx-auto w-full max-w-[760px] px-4 py-10 sm:px-6 sm:py-14">
+      <section aria-labelledby="page-title">
+        <p class="text-brand mb-2 text-sm font-bold">سرویس تبدیل کارت به شبا</p>
+        <h1 id="page-title" class="text-foreground text-2xl leading-[1.5] font-extrabold sm:text-3xl">
+          شماره شبا را سریع و مطمئن پیدا کنید
+        </h1>
+        <p class="text-muted mt-3 max-w-[620px] text-sm leading-8 sm:text-[15px]">
+          استعلام مستقیماً از سرویس زرین‌هاب انجام می‌شود. شماره کامل کارت در گزارش‌ها یا پاسخ عمومی نگهداری نمی‌شود.
+        </p>
+      </section>
+
+      <div class="mt-7 sm:mt-8">
+        <CardConverter :api-base-url="config.public.apiBaseUrl" />
+      </div>
+
+      <p class="text-muted mt-5 flex items-start gap-2 text-xs leading-6">
+        <svg viewBox="0 0 24 24" class="mt-0.5 size-4 shrink-0" fill="none" stroke="currentColor" stroke-width="1.8" aria-hidden="true">
+          <path d="M12 22s8-4 8-10V5l-8-3-8 3v7c0 6 8 10 8 10Z" />
+          <path d="m9 12 2 2 4-4" />
+        </svg>
+        برای حفاظت از اطلاعات بانکی، فقط نسخه پوشیده‌شده شماره کارت در سوابق فنی ثبت می‌شود.
+      </p>
+    </main>
+  </div>
+</template>