WebRTC TURN Preparation

Current repo truth

• durable call-session state remains in svc-messaging
• signaling remains on the existing websocket channel
• browser WebRTC peers now resolve ICE config from web env instead of ad hoc literals
• TURN remains optional for local development; STUN-only fallback still works

Environment contract

Frontend:

• NEXT_PUBLIC_WEBRTC_STUN_URLS
• NEXT_PUBLIC_WEBRTC_TURN_URLS
• NEXT_PUBLIC_WEBRTC_TURN_USERNAME
• NEXT_PUBLIC_WEBRTC_TURN_CREDENTIAL
• NEXT_PUBLIC_WEBRTC_ICE_TRANSPORT_POLICY

Backend / deploy placeholders:

• WEBRTC_STUN_URLS
• WEBRTC_TURN_URLS