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/packages/harness/deerflow/runtime/journal.py

@@ -386,13 +386,27 @@ class RunJournal(BaseCallbackHandler):
             return
         batch = self._buffer.copy()
         self._buffer.clear()
-        loop.create_task(self._flush_async(batch))
+        task = loop.create_task(self._flush_async(batch))
+        task.add_done_callback(self._on_flush_done)
 
     async def _flush_async(self, batch: list[dict]) -> None:
         try:
             await self._store.put_batch(batch)
         except Exception:
-            logger.warning("RunJournal: failed to flush %d events", len(batch), exc_info=True)
+            logger.warning(
+                "Failed to flush %d events for run %s — returning to buffer",
+                len(batch), self.run_id, exc_info=True,
+            )
+            # Return failed events to buffer for retry on next flush
+            self._buffer = batch + self._buffer
+
+    @staticmethod
+    def _on_flush_done(task: asyncio.Task) -> None:
+        if task.cancelled():
+            return
+        exc = task.exception()
+        if exc:
+            logger.warning("Journal flush task failed: %s", exc)
 
     def _identify_caller(self, kwargs: dict) -> str:
         for tag in kwargs.get("tags") or []: