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.
