Hook registers a 15-minute schedule that starts/resumes the backfill only
inside the maintenance window (no external cron or token). The backfill
pauses when the window closes and resumes next night. UI shows paused
state and last-run errors.
Pre-scan storage filenames and check each task's predicted cache file
before generating. Existing previews are now counted as "already cached"
and skipped, instead of being counted as remaining work and regenerated.
Storage-scan and plan-computation failures fall back to generation.
