docs: add built-in payment configuration guide and update README

- 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

README.md

@@ -42,8 +42,9 @@ Sub2API is an AI API gateway platform designed to distribute and manage API quot
 - **Smart Scheduling** - Intelligent account selection with sticky sessions
 - **Concurrency Control** - Per-user and per-account concurrency limits
 - **Rate Limiting** - Configurable request and token rate limits
+- **Built-in Payment System** - Supports EasyPay, Alipay, WeChat Pay, and Stripe for user self-service top-up, no separate payment service needed ([Configuration Guide](docs/PAYMENT.md))
 - **Admin Dashboard** - Web interface for monitoring and management
-- **External System Integration** - Embed external systems (e.g. payment, ticketing) via iframe to extend the admin dashboard
+- **External System Integration** - Embed external systems (e.g. ticketing) via iframe to extend the admin dashboard
 
 ## ❤️ Sponsors
 
@@ -88,7 +89,7 @@ Community projects that extend or integrate with Sub2API:
 
 | Project | Description | Features |
 |---------|-------------|----------|
-| [Sub2ApiPay](https://github.com/touwaeriol/sub2apipay) | Self-service payment system | Self-service top-up and subscription purchase; supports YiPay protocol, WeChat Pay, Alipay, Stripe; embeddable via iframe |
+| ~~[Sub2ApiPay](https://github.com/touwaeriol/sub2apipay)~~ | ~~Self-service payment system~~ | **Now Built-in** — Payment is now integrated into Sub2API, no separate deployment needed. See [Payment Configuration Guide](docs/PAYMENT.md) |
 | [sub2api-mobile](https://github.com/ckken/sub2api-mobile) | Mobile admin console | Cross-platform app (iOS/Android/Web) for user management, account management, monitoring dashboard, and multi-backend switching; built with Expo + React Native |
 
 ## Tech Stack