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/config.py

@@ -43,3 +43,13 @@ PORT = int(os.getenv("PORT", "8000"))
 CORS_ORIGINS = _split(
     os.getenv("CORS_ORIGINS", "http://localhost:3000,http://127.0.0.1:3000")
 )
+
+# ---- WebRTC TURN(公网跨网语音预览;本地开发留空即可) ----
+# TURN_URLS 示例:turn:182.92.86.220:3478?transport=udp,turn:182.92.86.220:3478?transport=tcp
+TURN_URLS = _split(os.getenv("TURN_URLS", ""))
+# coturn --use-auth-secret 时与 --static-auth-secret 一致;后端据此签发短时凭证
+TURN_SECRET = os.getenv("TURN_SECRET", "")
+# 不用 secret 时可改静态账号(需 coturn --user=...)
+TURN_USERNAME = os.getenv("TURN_USERNAME", "")
+TURN_PASSWORD = os.getenv("TURN_PASSWORD", "")
+TURN_CREDENTIAL_TTL = int(os.getenv("TURN_CREDENTIAL_TTL", "86400"))