M2b：删除旧 chat 体系——线性会话/叩心/情绪/纪念日模型与页面退役（迁移 003 双向验证），runs 去会话外键，测试全量修剪

backend/tests/conftest.py

@@ -74,16 +74,6 @@ async def reset_database() -> AsyncGenerator[None, None]:
     from app.services.ai_provider_admin import set_endpoint_overrides_cache
 
     set_endpoint_overrides_cache([])
-    # 清理可能被前序 TestClient 启动的 KnockingPivot 后台任务，避免写锁冲突
-    from app.main import app as _app
-
-    pivot = getattr(_app.state, "knocking_pivot", None)
-    if pivot is not None:
-        try:  # noqa: SIM105 — pivot.stop() 是协程，contextlib.suppress 不适用
-            await pivot.stop()
-        except Exception:
-            pass
-        _app.state.__dict__.pop("knocking_pivot", None)
     yield
     _assert_test_database_url(str(_engine.url))
     async with _engine.begin() as conn: