GitLab

- Fix webhook URLs: /payment/webhook/<provider> not /payment/<provider>/notify
- Remove notifyUrl/returnUrl from provider config tables (auto-generated by UI)
- Adjust wxpay publicKeyId/certSerial to match frontend optional marking

- Add docs/PAYMENT.md and docs/PAYMENT_CN.md with full payment setup guide
- Mark Sub2ApiPay as deprecated in ecosystem tables (payment is now built-in)
- Add built-in payment system to features list in all 3 READMEs