- /api/ice: when TURN_SECRET is set, mint short-lived HMAC credentials
  (coturn use-auth-secret) so no permanent password is exposed and the relay
  can't be abused. Static TURN_USERNAME/CREDENTIAL still supported.
- share.html: connection watchdog + clear "couldn't connect on this network"
  message instead of a blank screen when no path can be established.
- deploy/coturn: ready-to-run turnserver.conf + docker-compose + README for
  hosting our own TURN on a VM we own (flat cost, no per-GB billing).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>