fix confirmed deployment risks

backend/internal/handler/payment_webhook_handler.go

@@ -80,7 +80,7 @@ func (h *PaymentWebhookHandler) handleNotify(c *gin.Context, providerKey string)
 	provider, err := h.paymentService.GetWebhookProvider(c.Request.Context(), providerKey, outTradeNo)
 	if err != nil {
 		slog.Warn("[Payment Webhook] provider not found", "provider", providerKey, "outTradeNo", outTradeNo, "error", err)
-		writeSuccessResponse(c, providerKey)
+		writeUnavailableResponse(c, providerKey)
 		return
 	}
 
@@ -137,12 +137,22 @@ type wxpaySuccessResponse struct {
 	Message string `json:"message"`
 }
 
+type wxpayFailureResponse struct {
+	Code    string `json:"code"`
+	Message string `json:"message"`
+}
+
 // WeChat Pay webhook success response constants.
 const (
 	wxpaySuccessCode    = "SUCCESS"
 	wxpaySuccessMessage = "成功"
 )
 
+const (
+	wxpayFailureCode   = "FAIL"
+	unavailableMessage = "provider unavailable"
+)
+
 // writeSuccessResponse sends the provider-specific success response.
 // WeChat Pay requires JSON {"code":"SUCCESS","message":"成功"};
 // Stripe expects an empty 200; others accept plain text "success".
@@ -156,3 +166,14 @@ func writeSuccessResponse(c *gin.Context, providerKey string) {
 		c.String(http.StatusOK, "success")
 	}
 }
+
+func writeUnavailableResponse(c *gin.Context, providerKey string) {
+	switch providerKey {
+	case payment.TypeWxpay:
+		c.JSON(http.StatusServiceUnavailable, wxpayFailureResponse{Code: wxpayFailureCode, Message: unavailableMessage})
+	case payment.TypeStripe:
+		c.String(http.StatusServiceUnavailable, "")
+	default:
+		c.String(http.StatusServiceUnavailable, unavailableMessage)
+	}
+}