feat: async significance pass with auto-pin on score 3

tests/test_turn_flow.py

@@ -58,6 +58,11 @@ def client(tmp_path, monkeypatch):
     app.dependency_overrides[get_llm_client] = lambda: mock
 
     with TestClient(app) as c:
+        # Disable the lifespan-managed background worker — it would
+        # otherwise try to score significance through Featherless with
+        # a fake test API key. Worker behavior is exercised directly in
+        # tests/test_significance.py with a mock LLM factory.
+        app.state.background_worker.enabled = False
         c.mock_llm = mock  # type: ignore[attr-defined]
         yield c