fix(persistence): address review feedback on PR #1851

- Fix naive datetime.now() → datetime.now(UTC) in all ORM models
- Fix seq race condition in DbRunEventStore.put() with FOR UPDATE
  and UNIQUE(thread_id, seq) constraint
- Encapsulate _store access in RunManager.update_run_completion()
- Deduplicate _store.put() logic in RunManager via _persist_to_store()
- Add update_run_completion to RunStore ABC + MemoryRunStore
- Wire follow_up_to_run_id through the full create path
- Add error recovery to RunJournal._flush_sync() lost-event scenario
- Add migration note for search_threads breaking change
- Fix test_checkpointer_none_fix mock to set database=None

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/tests/test_checkpointer_none_fix.py

@@ -14,9 +14,10 @@ class TestCheckpointerNoneFix:
         """make_checkpointer should return InMemorySaver when config.checkpointer is None."""
         from deerflow.agents.checkpointer.async_provider import make_checkpointer
 
-        # Mock get_app_config to return a config with checkpointer=None
+        # Mock get_app_config to return a config with checkpointer=None and database=None
         mock_config = MagicMock()
         mock_config.checkpointer = None
+        mock_config.database = None
 
         with patch("deerflow.agents.checkpointer.async_provider.get_app_config", return_value=mock_config):
             async with make_checkpointer() as checkpointer: