feat(payment): Stripe Checkout payment module

- payment_orders 建表迁移，支持多 provider（stripe）
- Stripe Checkout Session 创建、Webhook 验签、到账
- 支付成功自动为 API Key 增加额度
- payment.enabled 开关，关闭时不影响旧 Comfy Checkout

src/services/mod.rs

@@ -5,6 +5,7 @@ pub mod comfyui_process;
 pub mod cron;
 pub mod notification;
 pub mod oauth;
+pub mod payment;
 pub mod register_captcha;
 pub mod registration_limiter;
 pub mod route_rebuild;