Python Chat Backend Implementation

I need a solid backend written in Python that powers one-on-one messaging for an existing mobile chat application. The core tasks are to expose clean REST and WebSocket endpoints, handle real-time delivery, persist messages reliably in PostgreSQL, and keep latency low under moderate load.

Authentication is already in place; your job starts at the moment the client is authorised. From there the service must:
• accept text, emoji, and small attachment payloads
• write each message atomically to Postgres, marking it delivered and later read
• push live updates back to the correct recipient through WebSockets
• expose a simple status route for health checks and basic metrics

Unit tests that cover the message flow, delivery confirmations, and failure scenarios are part of the definition of done. I am open to any Python framework you’re comfortable with—FastAPI, Django, or Flask—as long as the final code is clear, documented, and container-ready (Docker).

Please include a brief outline of your proposed architecture and any similar chat or real-time work you have shipped.