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/management/commands/discover_pobsync_snapshots.py

@@ -0,0 +1,39 @@
+from __future__ import annotations
+
+from typing import Any
+
+from django.core.management.base import BaseCommand, CommandError
+
+from pobsync.snapshot_meta import normalize_kind
+from pobsync_backend.models import GlobalConfig, HostConfig
+from pobsync_backend.snapshot_discovery import discover_snapshots
+
+
+class Command(BaseCommand):
+    help = "Discover snapshot metadata on disk and upsert SnapshotRecord rows."
+
+    def add_arguments(self, parser) -> None:
+        parser.add_argument("--host", default=None)
+        parser.add_argument("--kind", default="all", help="scheduled|manual|incomplete|all")
+
+    def handle(self, *args: Any, **options: Any) -> None:
+        try:
+            global_config = GlobalConfig.objects.get(name="default")
+        except GlobalConfig.DoesNotExist as exc:
+            raise CommandError("Missing GlobalConfig 'default'") from exc
+
+        host = None
+        if options["host"]:
+            try:
+                host = HostConfig.objects.get(host=options["host"], enabled=True)
+            except HostConfig.DoesNotExist as exc:
+                raise CommandError(f"Missing enabled HostConfig {options['host']!r}") from exc
+
+        kind = normalize_kind(options["kind"])
+        kinds = ["scheduled", "manual", "incomplete"] if kind == "all" else [kind]
+        result = discover_snapshots(host=host, global_config=global_config, kinds=kinds)
+        self.stdout.write(
+            self.style.SUCCESS(
+                f"Scanned {result['scanned']} snapshot(s), created {result['created']}, updated {result['updated']}."
+            )
+        )