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/ent/schema/user.go

@@ -87,6 +87,7 @@ func (User) Edges() []ent.Edge {
 		edge.To("usage_logs", UsageLog.Type),
 		edge.To("attribute_values", UserAttributeValue.Type),
 		edge.To("promo_code_usages", PromoCodeUsage.Type),
+		edge.To("payment_orders", PaymentOrder.Type),
 	}
 }