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/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"),
+};