- 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>