Self-heal stale local_path pointers on read

Backups completed before the keep-local-backups feature never had
local_path nulled out after the local archive was deleted, so the new
"Local copy: Kept" indicator was trusting a stale DB column instead of
reality. GET /api/backups and GET /api/backups/{id} now verify the
file still exists and clear local_path if it doesn't, which also
covers any future case of a local file disappearing outside the app.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/backup_service.py

@@ -130,11 +130,20 @@ def list_backups(guest_vmid: int | None = None, state: str | None = None) -> lis
         query += " WHERE " + " AND ".join(where)
     query += " ORDER BY started_at DESC LIMIT 500"
     with db() as conn:
-        return [dict(row) for row in conn.execute(query, args).fetchall()]
+        rows = [dict(row) for row in conn.execute(query, args).fetchall()]
+    return [_reconcile_local_path(row) for row in rows]
+
+
+def _reconcile_local_path(backup: dict) -> dict:
+    """Self-heal a stale local_path left over from a file that no longer exists."""
+    local_path = backup.get("local_path")
+    if local_path and not Path(local_path).exists():
+        return update_backup(backup["id"], local_path=None)
+    return backup
 
 
 def get_backup(backup_id: str) -> dict:
-    return _backup_row(backup_id)
+    return _reconcile_local_path(_backup_row(backup_id))
 
 
 def purge_deleted_backup(backup_id: str) -> dict: