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/service/settings_view.go

@@ -139,6 +139,7 @@ type PublicSettings struct {
 	BackendModeEnabled    bool
 	OIDCOAuthEnabled      bool
 	OIDCOAuthProviderName string
+	PaymentEnabled        bool
 	Version               string
 }