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",