feat(client): implement exponential backoff retry for network errors

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

@@ -79,6 +79,7 @@ func run(logger *slog.Logger) error {
 		httpClient,
 		config.ZarinHubEndpoint,
 		tokenManager,
+		config.ZarinHubRetries,
 	)
 	cardService := service.NewCardService(zarinHubClient)
 	auditRepository := repository.NewAuditRepository(db)