feat: 新增支付页面接口，为接入支付宝做准备

- GET /payment: 套餐选择页（无需认证，外部浏览器打开）
- GET /payment/page: 支付引导页（需 JWT，创建订单）
- GET /payment/pay: 支付宝电脑网站支付占位页（需 JWT）
- AppState 新增支付宝配置字段（可选）
- afterbody.js: PDF下载增加状态提示和重试按钮

src/handlers/mod.rs

@@ -31,6 +31,9 @@ pub use weather::get_weather_brief;
 pub use weather::get_weather_details;
 pub use weather::post_weather_data;
 
+pub use payment::alipay_trade_page_pay;
 pub use payment::create_order;
 pub use payment::get_user_quota;
 pub use payment::mock_confirm;
+pub use payment::payment_index;
+pub use payment::payment_page;