v2.2.1: frontend closed-source + Docker one-click deploy

- Remove frontend source code (now in private repo)
- Add pre-built frontend/dist/ with Nginx serving
- Simplify docker-compose.yml (no Node.js build needed)
- Update README with docs index and Docker deploy guide
- Add admin order list and AI analysis stats tabs
- Add quick trade API routes
- Clean up redundant files (package-lock.json, yarn.lock, .iml)
- Add GitHub Actions workflow for frontend update automation

backend_api_python/env.example

@@ -347,6 +347,8 @@ TRONGRID_API_KEY=
 # Order confirmation delay and expiration
 USDT_PAY_CONFIRM_SECONDS=30
 USDT_PAY_EXPIRE_MINUTES=30
+# Background worker poll interval (seconds) for checking pending USDT orders
+USDT_WORKER_POLL_INTERVAL=30
 
 # New user registration bonus credits (新用户注册赠送积分)
 CREDITS_REGISTER_BONUS=100