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.
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.
