Cron-triggered endpoint that ensures the right number of voice chatrooms exist for each community hour that's currently active or about to start.

Authenticated by the INTERNAL_CRON_TOKEN shared secret (2026-09-18 route-auth review); test_every_route_answers.py had it listed as "cron trigger anyone can fire". It creates voice chatrooms, so an open version is a room-spam button. kubernetes/cronjobs/community-rooms-sync.yaml now sends the header from the fluentea-secrets secret.

Logic per event:

1. Skip if we're not within the [start - lead, start + duration] window.
2. Count confirmed RSVPs for the upcoming/current occurrence.
3. Compute desired room count: max(1, ceil(rsvp_count / TARGET_PER_ROOM)).
4. Count existing chatrooms already auto-created for this occurrence (matched by activity_config.community_event_id).
5. Create the difference, all with activity_type=canasta and the right language.

Idempotent — safe to call repeatedly. Uses activity_config.community_event_id as the dedup key so we don't double-spawn within a single occurrence.