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.

frontend/src/hooks/use-voice-preview.ts

@@ -19,7 +19,7 @@
 
 import { useCallback, useEffect, useRef, useState } from "react";
 
-import { API_BASE } from "@/lib/api";
+import { API_BASE, webrtcApi } from "@/lib/api";
 
 export type VoicePreviewStatus = "idle" | "connecting" | "connected" | "failed";
 
@@ -301,10 +301,12 @@ export function useVoicePreview(
         if (wsRef.current === ws) closeOnRemoteEnd("语音信令连接已断开。");
       };
 
-      // 2) 建 PeerConnection(纯 STUN,本机/局域网够用)
-      const pc = new RTCPeerConnection({
-        iceServers: [{ urls: "stun:stun.l.google.com:19302" }],
-      });
+      // 2) 建 PeerConnection(STUN;公网跨网时后端会下发 TURN)
+      const iceServers = await webrtcApi
+        .iceServers()
+        .then((r) => r.iceServers)
+        .catch(() => [{ urls: "stun:stun.l.google.com:19302" }]);
+      const pc = new RTCPeerConnection({ iceServers });
       pcRef.current = pc;
 
       pc.onicecandidate = (e) => {

frontend/src/lib/api.ts

@@ -208,3 +208,14 @@ export type NodeTypesResponse = {
 export const nodeTypesApi = {
   list: () => request<NodeTypesResponse>("/api/node-types"),
 };
+
+export type IceServerConfig = {
+  urls: string | string[];
+  username?: string;
+  credential?: string;
+};
+
+export const webrtcApi = {
+  iceServers: () =>
+    request<{ iceServers: IceServerConfig[] }>("/api/webrtc/ice-servers"),
+};