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/cmd/server/wire.go

@@ -13,6 +13,7 @@ import (
 	"github.com/Wei-Shaw/sub2api/ent"
 	"github.com/Wei-Shaw/sub2api/internal/config"
 	"github.com/Wei-Shaw/sub2api/internal/handler"
+	"github.com/Wei-Shaw/sub2api/internal/payment"
 	"github.com/Wei-Shaw/sub2api/internal/repository"
 	"github.com/Wei-Shaw/sub2api/internal/server"
 	"github.com/Wei-Shaw/sub2api/internal/server/middleware"
@@ -41,6 +42,13 @@ func initializeApplication(buildInfo handler.BuildInfo) (*Application, error) {
 		// Server layer ProviderSet
 		server.ProviderSet,
 
+		// Payment providers
+		payment.ProvideRegistry,
+		payment.ProvideEncryptionKey,
+		payment.ProvideDefaultLoadBalancer,
+		service.ProvidePaymentConfigService,
+		service.ProvidePaymentOrderExpiryService,
+
 		// Privacy client factory for OpenAI training opt-out
 		providePrivacyClientFactory,
 
@@ -95,6 +103,7 @@ func provideCleanup(
 	openAIGateway *service.OpenAIGatewayService,
 	scheduledTestRunner *service.ScheduledTestRunnerService,
 	backupSvc *service.BackupService,
+	paymentOrderExpiry *service.PaymentOrderExpiryService,
 ) func() {
 	return func() {
 		ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
@@ -237,6 +246,12 @@ func provideCleanup(
 				}
 				return nil
 			}},
+			{"PaymentOrderExpiryService", func() error {
+				if paymentOrderExpiry != nil {
+					paymentOrderExpiry.Stop()
+				}
+				return nil
+			}},
 		}
 
 		infraSteps := []cleanupStep{