[Refactor] Remove unused Session presence tracking

[roomote-roomote]

​Created by Roomote. View the task or mention @roomote-roomote for follow-up asks.

What changed

• Removed the Session viewer heartbeat hook and its focus, visibility, page lifecycle, and timer handling.
• Removed the authenticated presence API route and the Redis lease implementation and exports.
• Removed the dedicated tests for the deleted lifecycle while preserving the shared Redis client structure used elsewhere.

Why this change was made

Session presence was continuously written from open browser tabs, but no production code consumed the presence query. Keeping the distributed browser, API, and Redis lifecycle imposed ongoing network and maintenance cost without affecting product behavior.

Impact

Opening a Session no longer starts background presence requests or creates Redis leases. There is no intended user-facing behavior change because notification routing and other production paths never read the presence signal.

[roomote-community]

No code issues found. See task

_{Reviewed 93ed1da}