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/tests/test_backup_service.py

@@ -573,6 +573,40 @@ def test_retention_delete_also_removes_local_copy(
     assert final["local_path"] is None
 
 
+def test_get_and_list_backups_clear_stale_local_path_for_missing_file(isolated_db, no_notifications, tmp_path):
+    from app import backup_service
+
+    missing_local = tmp_path / "gone.tar.zst"
+
+    with db() as conn:
+        conn.execute(
+            """
+            INSERT INTO backups(
+                id, guest_vmid, guest_name, guest_type, node, state,
+                local_path, remote_path, started_at, completed_at, updated_at
+            )
+            VALUES ('stale-local', 110, 'test-guest', 'vm', 'pve', 'completed',
+                    ?, 'onedrive:pve-cloud-backup/stale-local/archive.tar.zst', ?, ?, ?)
+            """,
+            (str(missing_local), utc_now(), utc_now(), utc_now()),
+        )
+
+    fetched = backup_service.get_backup("stale-local")
+    assert fetched["local_path"] is None
+
+    with db() as conn:
+        row = conn.execute("SELECT local_path FROM backups WHERE id = 'stale-local'").fetchone()
+    assert row["local_path"] is None
+
+    with db() as conn:
+        conn.execute(
+            "UPDATE backups SET local_path = ? WHERE id = 'stale-local'",
+            (str(missing_local),),
+        )
+    listed = backup_service.list_backups()
+    assert listed[0]["local_path"] is None
+
+
 def test_recover_unfinished_resumes_local_downloading(isolated_db, no_notifications, monkeypatch):
     from app import backup_service