`/chat` endpoint needs proper handling of long-running task

[tonyzhao6]

The /chat endpoint is currently using FastAPI's background task to execute the search process and LLM calls in order to get around Turn.io's 20 second limitation. This is not a robust solution since FastAPI's background task does not guarantee execution order.

Suitable solutions for productions are:

1. Websockets (but Turn.io does not support this currently)
2. Proper message queue system like Celery, RMQ, etc.
3. Preventing users from requesting another message until the current background task finishes.

NB: This does not affect the frontend UI since there is no way to call the /chat endpoint with a WA ID and Turn API key from the UI.