fix: enable built-in TURN server for NAT traversal

Subscriber transport was timing out on DTLS handshake for clients
behind complex NAT (VPN/symmetric NAT). Enable LiveKit's built-in
TURN server on UDP port 3478.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

deploy/docker/docker-compose.yml

@@ -320,6 +320,7 @@ services:
     ports:
       - "17880:7880"
       - "17881:7881"
+      - "3478:3478/udp"
       - "50000-50200:50000-50200/udp"
     volumes:
       - ./livekit.yaml:/etc/livekit.yaml:ro

deploy/docker/livekit.yaml

@@ -5,6 +5,11 @@ rtc:
   tcp_port: 7881
   use_external_ip: true
 
+turn:
+  enabled: true
+  udp_port: 3478
+  tls_port: 0
+
 keys:
   devkey: devsecret