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_gen_test.go

@@ -75,6 +75,7 @@ func TestProvideCleanup_WithMinimalDependencies_NoPanic(t *testing.T) {
 		nil, // openAIGateway
 		nil, // scheduledTestRunner
 		nil, // backupSvc
+		nil, // paymentOrderExpiry
 	)
 
 	require.NotPanics(t, func() {