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.

src/pobsync_backend/tests/test_console_entrypoint.py

@@ -39,3 +39,10 @@ class ConsoleEntrypointTests(SimpleTestCase):
         execute.assert_called_once_with(
             ["pobsync", "configure_pobsync_schedule", "web-01", "--cron", "15 2 * * *"]
         )
+
+    def test_maps_discover_snapshots_alias_to_django_command(self) -> None:
+        with patch("pobsync.cli.execute_from_command_line") as execute:
+            exit_code = main(["discover-snapshots", "--host", "web-01"])
+
+        self.assertEqual(exit_code, 0)
+        execute.assert_called_once_with(["pobsync", "discover_pobsync_snapshots", "--host", "web-01"])