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/.env.example

@@ -31,5 +31,12 @@ DATABASE_URL=postgresql+asyncpg://postgres:postgres@localhost:5432/postgres
 # ---- 服务监听 & 跨域 ----
 HOST=0.0.0.0
 PORT=8000
-# 前端开发地址,允许跨域
+# 前端开发地址,允许跨域(公网部署时加上实际前端 origin)
 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) ----
+# 与 docker compose --profile remote 的 coturn 配套。云安全组需放行 UDP 3478 与 49152-49200。
+# PUBLIC_IP 填云主机公网 IP(compose 里给 coturn --external-ip 用,见项目根 .env)。
+# TURN_URLS=turn:182.92.86.220:3478?transport=udp,turn:182.92.86.220:3478?transport=tcp
+# TURN_SECRET=your-turn-secret
+# TURN_CREDENTIAL_TTL=86400