feat: Redis-backed session store for chat history

[alexandervazquez98]

Context

The current chat endpoint in �ackend/routers/chat.py uses an in-memory Python dict (_SESSION_STORE) with a 60-minute TTL. This means:

• Chat history is lost on every process restart
• History is not shared across multiple backend instances

Proposed Solution

Replace _SESSION_STORE with Redis-backed sessions using the existing
edis_client.py:

1. Store each session as a Redis key: chat:session:{session_id}
2. Use Redis TTL (60 min) instead of manual cleanup task
3. Serialize messages as JSON in Redis values
4. Remove the cleanup_expired_sessions() coroutine and lifespan task

Acceptance Criteria

• Chat sessions survive backend process restart
• Sessions auto-expire after 60 minutes via Redis TTL
• Graceful fallback if Redis is unavailable
• All existing chat tests updated and passing
• No changes to the HTTP API contract

Notes

Redis is already available in the stack (
edis_client.py handles the connection). P2 item carried over from �ugfix-code-review.