Migrate Response API to Celery for Background Processing

[avirajsingh7]

Currently, the Response API uses FastAPI’s built-in background tasks, which run on the same machine as the main API process. This causes resource contention and limits scalability.

We should migrate response handling to Celery, so responses are generated in distributed worker processes. This ensures API requests return quickly, while heavy response generation is processed asynchronously.

Also, Introduce a Job model to track task status (PENDING → PROCESSING → SUCCESS/FAILED).