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/services/webrtc_ice.py

@@ -0,0 +1,60 @@
+"""WebRTC ICE server config: STUN + optional TURN for cross-network voice preview."""
+
+from __future__ import annotations
+
+import base64
+import hashlib
+import hmac
+import time
+
+import config
+
+STUN_URL = "stun:stun.l.google.com:19302"
+
+
+def _turn_credentials() -> tuple[str, str] | None:
+    """Return (username, credential) for TURN, or None when TURN is not configured."""
+    if not config.TURN_URLS:
+        return None
+    if config.TURN_SECRET:
+        expiry = int(time.time()) + config.TURN_CREDENTIAL_TTL
+        username = f"{expiry}:ai-video"
+        credential = base64.b64encode(
+            hmac.new(
+                config.TURN_SECRET.encode("utf-8"),
+                username.encode("utf-8"),
+                hashlib.sha1,
+            ).digest()
+        ).decode("utf-8")
+        return username, credential
+    if config.TURN_USERNAME and config.TURN_PASSWORD:
+        return config.TURN_USERNAME, config.TURN_PASSWORD
+    return None
+
+
+def client_ice_servers() -> list[dict]:
+    """ICE servers for browser RTCPeerConnection (JSON-serializable)."""
+    servers: list[dict] = [{"urls": STUN_URL}]
+    creds = _turn_credentials()
+    if not creds:
+        return servers
+    username, credential = creds
+    for url in config.TURN_URLS:
+        servers.append({"urls": url, "username": username, "credential": credential})
+    return servers
+
+
+def aiortc_ice_servers() -> list:
+    """ICE servers for backend SmallWebRTCConnection / aiortc."""
+    from aiortc import RTCIceServer
+
+    servers = [RTCIceServer(urls=STUN_URL)]
+    creds = _turn_credentials()
+    if not creds:
+        return servers
+    username, credential = creds
+    for url in config.TURN_URLS:
+        servers.append(
+            RTCIceServer(urls=url, username=username, credential=credential)
+        )
+    return servers