Today's Garage outage at 02:00-04:00 UTC silently lost 3 of 6 daily
backups (paperless, audiobookshelf, authentik all FailedValidation
because BackupStorageLocation was Unavailable). GarageDown alert
fired, but nothing surfaced per-schedule backup loss. Add three
alerts covering validation failure, failed/partial backup, and
30h+ stale schedule.