feat(threads): switch search endpoint to threads_meta table and sync title

- POST /api/threads/search now queries threads_meta table directly,
  removing the two-phase Store + Checkpointer scan approach
- Add ThreadMetaRepository.search() with metadata/status filters
- Add ThreadMetaRepository.update_display_name() for title sync
- Worker syncs checkpoint title to threads_meta.display_name on run completion
- Map display_name to values.title in search response for API compatibility

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

backend/app/gateway/services.py

@@ -323,6 +323,7 @@ async def start_run(
             event_store=event_store,
             run_events_config=run_events_config,
             follow_up_to_run_id=follow_up_to_run_id,
+            thread_meta_repo=thread_meta_repo,
         )
     )
     record.task = task