fix: guard against accidental payment config wipe + cleanup from review

- Skip UpdatePaymentConfig when no payment fields provided (prevents wipe)
- Remove unused defaultMinRechargeAmount/defaultMaxRechargeAmount constants
- Fix mergeConfig comment to match actual behavior

backend/cmd/server/VERSION

@@ -1 +1 @@
-0.1.108.60
+0.1.108.61

backend/internal/handler/admin/setting_handler.go

@@ -660,8 +660,9 @@ func (h *SettingHandler) UpdateSettings(c *gin.Context) {
 		return
 	}
 
-	// Update payment configuration (integrated into system settings)
-	if h.paymentConfigService != nil {
+	// Update payment configuration (integrated into system settings).
+	// Skip if no payment fields were provided (prevents accidental wipe).
+	if h.paymentConfigService != nil && hasPaymentFields(req) {
 		paymentReq := service.UpdatePaymentConfigRequest{
 			Enabled:             req.PaymentEnabled,
 			MinAmount:           req.PaymentMinAmount,
@@ -780,6 +781,17 @@ func (h *SettingHandler) UpdateSettings(c *gin.Context) {
 	})
 }
 
+// hasPaymentFields returns true if any payment-related field was explicitly provided.
+func hasPaymentFields(req UpdateSettingsRequest) bool {
+	return req.PaymentEnabled != nil || req.PaymentMinAmount != nil ||
+		req.PaymentMaxAmount != nil || req.PaymentDailyLimit != nil ||
+		req.PaymentOrderTimeoutMin != nil || req.PaymentMaxPendingOrders != nil ||
+		req.PaymentEnabledTypes != nil || req.PaymentBalanceDisabled != nil ||
+		req.PaymentLoadBalanceStrat != nil || req.PaymentProductNamePrefix != nil ||
+		req.PaymentProductNameSuffix != nil || req.PaymentHelpImageURL != nil ||
+		req.PaymentHelpText != nil
+}
+
 func (h *SettingHandler) auditSettingsUpdate(c *gin.Context, before *service.SystemSettings, after *service.SystemSettings, req UpdateSettingsRequest) {
 	if before == nil || after == nil {
 		return

backend/internal/service/payment_config_service.go

@@ -38,10 +38,8 @@ const (
 
 // Default values for payment configuration settings.
 const (
-	defaultMinRechargeAmount = 1
-	defaultMaxRechargeAmount = 99999999.99
-	defaultOrderTimeoutMin   = 30
-	defaultMaxPendingOrders  = 3
+	defaultOrderTimeoutMin  = 30
+	defaultMaxPendingOrders = 3
 )
 
 // PaymentConfig holds the payment system configuration.
@@ -464,8 +462,9 @@ func (s *PaymentConfigService) UpdateProviderInstance(ctx context.Context, id in
 	return u.Save(ctx)
 }
 
-// mergeConfig merges new config with existing config, preserving sensitive
-// fields that were not re-submitted (frontend skips masked ••••••••).
+// mergeConfig merges new config with existing config. All keys from newConfig
+// overwrite existing values (including empty strings to clear a field).
+// Keys not present in newConfig are preserved from existing config.
 func (s *PaymentConfigService) mergeConfig(ctx context.Context, id int64, newConfig map[string]string) (map[string]string, error) {
 	inst, err := s.entClient.PaymentProviderInstance.Get(ctx, id)
 	if err != nil {