GitLab

Restructure the EasyPay recommendation block to present two options side
by side so users can pick by funding channel and settlement currency:

- Domestic / CNY — ZPay: official Alipay/WeChat API with 1.6% fee and
  T+1 automatic settlement (existing recommendation, expanded with fee
  and settlement details).
- International / USDT or USD — Kyren Topup (https://kyren.top): global
  payment stack supporting WeChat Pay and Alipay with local-currency
  checkout, USD settlement, and USDT/USD withdrawal. Fees: WeChat 2%,
  Alipay 2.5%, withdrawal 0.1% ($40 min / $150 max). Fills the gap for
  users who cannot use domestic Chinese channels or tolerate Stripe's
  6%+ fees.

Both recommendations share a single disclaimer at the end. The Chinese
heading uses "易支付" while the English one keeps "EasyPay".

- 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