Prosody and web client now receive TURN configuration (host, port, credentials)
enabling relay-based NAT traversal for mobile clients on symmetric NAT (4G/5G).
Coturn is shared from jami-infrastructure as jeffsi-coturn. Added coturn config
for reference. Also updated custom-config.js with TURN ICE servers for P2P.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>