Class EvictionMonitor

Fans account-wide eviction notifications out to per-devbox callbacks.

One monitor is shared by every Devbox built from the same generated client (see getEvictionMonitor), so all registered devboxes are served by a single SSE connection. The stream opens the moment the first devbox registers interest and closes as soon as the last interested devbox has been notified.

Delivery contract:

• The server replays every currently-pending eviction on connect, so a devbox that registers after its eviction was scheduled is still notified.
• Notifications for devboxes not in the interest set are discarded.
• A devbox is removed from the interest set before its callback runs, so the callback fires at most once even if the server repeats the notification.