fix: decouple import errors & fix history API to query langgraph_chat_histories

- experiment_log_route: self-contained _get_pool/_now (remove notes_route dep)
- roadmap_flow_route: self-contained _get_pool/_now (remove notes_route dep)
- check_history_route: query langgraph_chat_histories via async pool
  (was incorrectly querying dashboard_canifa.chat_history with wrong columns)
- real_user_history/check_history_route: same fix
- Verified: identity 468707 returns full chat history with products