Write a horizontal scaling guide (multi-worker + sticky sessions)

## Summary

Production compose uses one uvicorn worker. `docs/deployment/operations.md` has three bullets. Roadmap v1.1 wants a real scaling guide.

## Context

- Known limitation: single uvicorn worker
- WebSocket voice needs sticky sessions if you add replicas
- Postgres/Redis should move to managed services before app replicas

## Scope

New doc e.g. `docs/deployment/scaling.md` covering:
- Why `--workers 1` today
- Sticky sessions / WebSocket routing
- Session state in Redis vs in-process
- Knowledge worker and LiveKit worker as separate services
- What breaks if you naively set workers > 1

Link it from operations.md, deployment README, and known-limitations.

## Out of scope

Implementing multi-instance production in this repo’s default compose.

Start with docs/deployment/operations.md, the deployment README, and the known-limitations documentation to understand the current single-worker setup and existing links. Create the scaling guide covering the listed worker, session, routing, and service-separation topics, then link it from all three locations. Done means the documentation explains the multi-worker risks and scaling prerequisites without changing the default compose deployment.

Written by the indexing model from the issue text.