Admin: force every transcription client in a room to rebuild its pipeline.

Closes the app↔backend stream WebSocket for each session in the room; each client's health watchdog sees the closed socket and rebuilds (reconnect + fresh audio-processor tap). This is the targeted 'toggle' that did not exist during the 2026-07-15 class, when the only lever was a full pod restart that stranded arriving students. Deepgram-side sessions are destroyed too so the rebuilt clients start clean rather than evicting themselves.