test(db): lock probes read only their own database's locks

pg_locks is the whole server's: under the parallel lane a worker's own advisory claim on the same playlist id, or its own lock wait, was counted by another worker's probe (a leak reported where there was none, a commit made before the second tab blocked).

backend/tests/test_playlist_sync_claim.py

@@ -89,9 +89,13 @@ def test_a_claim_is_released_when_its_block_raises(db, make_user):
     except RuntimeError:
         pass
     # Asked of pg_locks, not by claiming again: a lock leaked on a pooled connection would be granted
-    # again to that same connection (advisory locks are re-entrant per session).
+    # again to that same connection (advisory locks are re-entrant per session). pg_locks is the whole
+    # server's, so only this database's: a parallel worker's own claim on the same id counted here.
     held = db.scalar(
-        text("SELECT count(*) FROM pg_locks WHERE locktype = 'advisory' AND objid = :id"),
+        text(
+            "SELECT count(*) FROM pg_locks WHERE locktype = 'advisory' AND objid = :id"
+            " AND database = (SELECT oid FROM pg_database WHERE datname = current_database())"
+        ),
         {"id": pl.id},
     )
     assert held == 0