Add WebRTC TURN configuration and related enhancements

- Introduce a new `.env.example` file for environment variable setup, including `PUBLIC_IP`, `TURN_SECRET`, and `TURN_URLS` for WebRTC TURN server configuration.
- Update `docker-compose.yaml` to support TURN server deployment with necessary environment variables and commands.
- Enhance backend configuration and routes to include WebRTC ICE server settings, allowing for STUN/TURN server integration.
- Implement a new service for managing WebRTC ICE server configurations, providing credentials for TURN when configured.
- Modify frontend API to fetch ICE server configurations dynamically, improving support for cross-network voice preview.

backend/app.py

@@ -9,6 +9,7 @@
   /api/model-resources 模型资源 CRUD
   /ws/voice            WebRTC 输出(浏览器)
   /ws/stream           WS 输出(裸音频流)
+  /api/webrtc/ice-servers  WebRTC STUN/TURN 配置
 """
 
 from contextlib import asynccontextmanager