feat(alert): add alertcooldown to limit repeated monitor alerts (#5211)
Add an optional alertcooldown setting, in minutes. When set, only the first monitor-session or monitor-query alert in each window is sent and the rest are logged as skipped. The monitor still restarts the server on every run.
The last sent time is stored in lock/<selfname>-alert-monitor.lock. Defaults to 0, which sends every alert as before.