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

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)
+		}
+	}
+}