feat: discover snapshots into Django records

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.

README.md

@@ -74,6 +74,12 @@ pobsync retention <host>
 pobsync retention <host> --apply --yes --max-delete 10
 ```
 
+Discover snapshots already present on disk:
+
+```
+pobsync discover-snapshots --host <host>
+```
+
 The `pobsync` executable is a thin wrapper around Django management commands. Direct Django access is also available:
 
 ```
@@ -143,6 +149,7 @@ The remaining internal engine code still contains reusable backup primitives:
 Next refactor targets:
 
 - Record discovered snapshots into `SnapshotRecord`.
+- Surface `SnapshotRecord` data through API/admin views instead of filesystem inspection.
 - Move more snapshot lifecycle details into typed domain objects.
 - Replace remaining dictionary-shaped config at engine boundaries.
 - Remove legacy YAML import/export once production migration no longer needs it.