When a remote relay's room goes inactive (all participants left),
the receiving relay now:
1. Clears remote_participants for that peer+room
2. Broadcasts updated RoomUpdate to local clients with the remote
participant removed
3. Updates federation_active_rooms metric
Previously, remote participants lingered in the participant list
after disconnect, causing ghost entries and stale media forwarding.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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