fix: add autouse fixture to clear dependency_overrides after each test

backend/tests/conftest.py

@@ -26,6 +26,14 @@ pytest_plugins = [
 ]
 
 
+@pytest.fixture(autouse=True)
+def _cleanup_dependency_overrides():
+    """Ensure dependency_overrides are cleaned up after each test."""
+    from app.main import app
+    yield
+    app.dependency_overrides.clear()
+
+
 @pytest.fixture(autouse=True)
 def add_api_key_filter():
     root_logger = logging.getLogger()