Jingle A/V (XEP-0166/0167/0176) needs a STUN/TURN server for NAT
traversal. Add coturn and advertise it to clients via XEP-0215.
- coturn container, host networking (a relay needs its full UDP port
range reachable, and Docker NAT hides the peer addresses coturn needs).
- mod_turn_external in Prosody to advertise the service and mint
time-limited credentials from a shared secret.
- Advertise UDP and TCP transports plus turns: on 5349, so clients on
UDP-blocking networks can still connect.
- Deny relaying to private ranges, so this is not an open proxy into
internal services.
TURN host is guschin.info rather than turn.guschin.info because only the
former is in the certificate SANs and turns: clients validate the name.
TURN_HOST/TURN_SECRET go in .env, which is gitignored and set per
deployment; TURN_SECRET must match static-auth-secret in turnserver.conf.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
