refactor(gateway): route all thread metadata access through ThreadMetaStore

Following the rename/delete bug fix in PR1, migrate the remaining direct
LangGraph Store reads/writes in the threads router and services to the
ThreadMetaStore abstraction so that the sqlite and memory backends behave
identically and the legacy dual-write paths can be removed.

Migrated endpoints (threads.py):
- create_thread: idempotency check + write now use thread_meta_repo.get/create
  instead of dual-writing the LangGraph Store and the SQL row.
- get_thread: reads from thread_meta_repo.get; the checkpoint-only fallback
  for legacy threads is preserved.
- patch_thread: replaced _store_get/_store_put with thread_meta_repo.update_metadata.
- delete_thread_data: dropped the legacy store.adelete; thread_meta_repo.delete
  already covers it.

Removed dead code (services.py):
- _upsert_thread_in_store — redundant with the immediately following
  thread_meta_repo.create() call.
- _sync_thread_title_after_run — worker.py's finally block already syncs
  the title via thread_meta_repo.update_display_name() after each run.

Removed dead code (threads.py):
- _store_get / _store_put / _store_upsert helpers (no remaining callers).
- THREADS_NS constant.
- get_store import (router no longer touches the LangGraph Store directly).

New abstract method:
- ThreadMetaStore.update_metadata(thread_id, metadata) merges metadata into
  the thread's metadata field. Implemented in both ThreadMetaRepository (SQL,
  read-modify-write inside one session) and MemoryThreadMetaStore. Three new
  unit tests cover merge / empty / nonexistent behaviour.

Net change: -134 lines. Full test suite: 1693 passed, 14 skipped.
Verified end-to-end with curl in gateway mode against sqlite backend
(create / patch / get / rename / search / delete).

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

backend/packages/harness/deerflow/persistence/thread_meta/sql.py

@@ -116,6 +116,22 @@ class ThreadMetaRepository(ThreadMetaStore):
             await session.execute(update(ThreadMetaRow).where(ThreadMetaRow.thread_id == thread_id).values(status=status, updated_at=datetime.now(UTC)))
             await session.commit()
 
+    async def update_metadata(self, thread_id: str, metadata: dict) -> None:
+        """Merge ``metadata`` into ``metadata_json``.
+
+        Read-modify-write inside a single session/transaction so concurrent
+        callers see consistent state. No-op if the row does not exist.
+        """
+        async with self._sf() as session:
+            row = await session.get(ThreadMetaRow, thread_id)
+            if row is None:
+                return
+            merged = dict(row.metadata_json or {})
+            merged.update(metadata)
+            row.metadata_json = merged
+            row.updated_at = datetime.now(UTC)
+            await session.commit()
+
     async def delete(self, thread_id: str) -> None:
         async with self._sf() as session:
             row = await session.get(ThreadMetaRow, thread_id)