refactor(payment): code standards fixes and regression repairs

Backend:
- Split payment_order.go (546→314 lines) into payment_order_lifecycle.go
- Replace magic strings with constants in factory, easypay, webhook handler
- Add rate limit/validity unit constants in payment_order_lifecycle, payment_service
- Fix critical regression: add PaymentEnabled to GetPublicSettings response
- Add missing migration 099_fix_migrated_purchase_menu_label_icon.sql

Frontend:
- Fix StripePopupView.vue: replace `as any` with typed interface, use extractApiErrorMessage
- Fix AdminOrderTable.vue: replace hardcoded column labels with i18n t() calls
- Fix SubscriptionsView.vue: replace hardcoded Today/Tomorrow with i18n
- Extract duplicate statusBadgeClass/canRefund/formatOrderDateTime to orderUtils.ts
- Add missing i18n keys: common.today, common.tomorrow, payment.orders.orderType/actions
- Remove dead PurchaseSubscriptionView.vue (replaced by PaymentView)

backend/internal/handler/payment_webhook_handler.go

@@ -137,13 +137,19 @@ type wxpaySuccessResponse struct {
 	Message string `json:"message"`
 }
 
+// WeChat Pay webhook success response constants.
+const (
+	wxpaySuccessCode    = "SUCCESS"
+	wxpaySuccessMessage = "成功"
+)
+
 // 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".
 func writeSuccessResponse(c *gin.Context, providerKey string) {
 	switch providerKey {
 	case payment.TypeWxpay:
-		c.JSON(http.StatusOK, wxpaySuccessResponse{Code: "SUCCESS", Message: "成功"})
+		c.JSON(http.StatusOK, wxpaySuccessResponse{Code: wxpaySuccessCode, Message: wxpaySuccessMessage})
 	case payment.TypeStripe:
 		c.String(http.StatusOK, "")
 	default: