Add a discovery preflight that reports the configured backup root, host
root, and snapshot directory counts before importing anything.
Show discovery status on host detail pages so missing mounts or mismatched
host directories are visible from the UI.
Warn clearly when discovery scans zero snapshots, including whether the
host backup directory is missing or simply empty.
Upsert SnapshotRecord rows directly from run_pobsync_backup results so
new successful and failed backup runs are reflected in the database
without requiring a separate discovery pass. Keep discovery for existing
snapshots and repair workflows, and cover success, failure, and dry-run
behavior with tests.
Add a Django-native snapshot discovery service and management command
that scans backup directories, reads snapshot metadata, and idempotently
upserts SnapshotRecord rows. Expose it through the pobsync command
wrapper, update admin/docs, and cover discovery behavior with tests.
