feat(payment): add complete payment system with multi-provider support

Add a full payment and subscription system supporting EasyPay (Alipay/WeChat),
Stripe, and direct Alipay/WeChat Pay providers with multi-instance load balancing.

backend/internal/server/router.go

@@ -111,4 +111,5 @@ func registerRoutes(
 	routes.RegisterUserRoutes(v1, h, jwtAuth, settingService)
 	routes.RegisterAdminRoutes(v1, h, adminAuth)
 	routes.RegisterGatewayRoutes(r, h, apiKeyAuth, apiKeyService, subscriptionService, opsService, settingService, cfg)
+	routes.RegisterPaymentRoutes(v1, h.Payment, h.PaymentWebhook, h.Admin.Payment, jwtAuth, adminAuth, settingService)
 }