feat(saas): add payment integration with Alipay/WeChat mock support

- payment.rs: create_payment, handle_payment_callback, query_payment_status
- Mock pay page for development mode with HTML confirm/cancel flow
- Payment callback handler with subscription auto-creation on success
- Alipay form-urlencoded and WeChat JSON callback parsing
- 7 new routes including callback and mock-pay endpoints

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/zclaw-saas/src/billing/mod.rs

@@ -1,10 +1,11 @@
-//! 计费模块 — 计划管理、订阅、用量配额
+//! 计费模块 — 计划管理、订阅、用量配额、支付
 
 pub mod types;
 pub mod service;
 pub mod handlers;
+pub mod payment;
 
-use axum::routing::get;
+use axum::routing::{get, post};
 
 pub fn routes() -> axum::Router<crate::state::AppState> {
     axum::Router::new()
@@ -12,4 +13,15 @@ pub fn routes() -> axum::Router<crate::state::AppState> {
         .route("/api/v1/billing/plans/{id}", get(handlers::get_plan))
         .route("/api/v1/billing/subscription", get(handlers::get_subscription))
         .route("/api/v1/billing/usage", get(handlers::get_usage))
+        .route("/api/v1/billing/payments", post(handlers::create_payment))
+        .route("/api/v1/billing/payments/{id}", get(handlers::get_payment_status))
+        // 支付回调（无需 auth）
+        .route("/api/v1/billing/callback/{method}", post(handlers::payment_callback))
+}
+
+/// mock 支付页面路由（开发模式）
+pub fn mock_routes() -> axum::Router<crate::state::AppState> {
+    axum::Router::new()
+        .route("/api/v1/billing/mock-pay", get(handlers::mock_pay_page))
+        .route("/api/v1/billing/mock-pay/confirm", post(handlers::mock_pay_confirm))
 }