Add activity logging to all write API routes, notification system with
real-time delivery via Socket.IO, @mention parsing in comments, activity
feed sidebar on board page, per-card activity in card modal, and
notification bell with unread badge in the navbar.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Wire Socket.IO broadcast from all write API routes so board changes
propagate instantly to other connected clients. Add session-based auth
on socket connections, in-memory presence tracking, and a presence UI
in the board header showing who else is viewing.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>