feat: link backup runs to snapshot records

Add a nullable SnapshotRecord foreign key to BackupRun and populate it
when run_pobsync_backup records a completed or failed snapshot. Keep the
existing snapshot_path for audit compatibility while making run-to-snapshot
navigation explicit in the database and admin.

src/pobsync_backend/management/commands/run_pobsync_backup.py

@@ -64,23 +64,26 @@ class Command(BaseCommand):
         rsync = result.get("rsync") if isinstance(result.get("rsync"), dict) else {}
         run.rsync_exit_code = rsync.get("exit_code")
         run.result = result
+        snapshot_record = None
+        if run.snapshot_path:
+            snapshot_path = Path(run.snapshot_path)
+            try:
+                kind = infer_snapshot_kind(snapshot_path)
+                snapshot_record, _created = upsert_snapshot_record(host=host, kind=kind, snapshot_dir=snapshot_path)
+            except ValueError:
+                snapshot_record = None
+        run.snapshot = snapshot_record
         run.save(
             update_fields=[
                 "status",
                 "ended_at",
                 "snapshot_path",
+                "snapshot",
                 "base_path",
                 "rsync_exit_code",
                 "result",
             ],
         )
-        if run.snapshot_path:
-            snapshot_path = Path(run.snapshot_path)
-            try:
-                kind = infer_snapshot_kind(snapshot_path)
-                upsert_snapshot_record(host=host, kind=kind, snapshot_dir=snapshot_path)
-            except ValueError:
-                pass
 
         if result.get("ok"):
             self.stdout.write(self.style.SUCCESS(f"Backup completed for {host.host}."))