refactor: make mock_enqueue_job fixture opt-in instead of autouse

Tests that call POST /api/counter/increment now explicitly request
the mock_enqueue_job fixture. This prevents the mock from masking
issues in other tests that don't need it.

backend/tests/conftest.py

@@ -241,13 +241,15 @@ async def user_no_roles(client_factory):
     }
 
 
-@pytest.fixture(autouse=True)
+@pytest.fixture
 def mock_enqueue_job():
-    """Mock job enqueueing for all tests.
+    """Mock job enqueueing for tests that hit the counter increment endpoint.
 
     pgqueuer requires PostgreSQL-specific features that aren't available
     in the test database setup. We mock the enqueue function to avoid
     connection issues while still testing the counter logic.
+
+    Tests that call POST /api/counter/increment must use this fixture.
     """
     mock = AsyncMock(return_value=1)  # Return a fake job ID
     with patch("routes.counter.enqueue_random_number_job", mock):