(release) Add purged snapshot audit overview

Record snapshot purge history whenever retention or incomplete cleanup removes
snapshot directories and SQL records. Store the purge reason, original kind,
path, action source, and triggering operator so manual, scheduled, CLI, and
incomplete cleanup actions remain auditable after the original snapshot record
is deleted.

Add a staff-only Purged Snapshots page with host/action filters and register
the audit model in Django admin.

Refs #16
Refs #8

src/pobsync_backend/backup_runner.py

@@ -109,6 +109,7 @@ def execute_backup_run(
                 protect_bases=bool(prune_protect_bases),
                 yes=True,
                 max_delete=int(prune_max_delete),
+                action=run.run_type,
                 acquire_lock=False,
             )
         except Exception as exc: