[BUG] Lite alerts re-fire after an app restart — email cooldown is in-memory only #981

Split out from #976. The reporter saw the same alerts pop up several times, hours after the originating event.

EmailAlertService deduplicates alert emails with an in-memory ConcurrentDictionary<string, DateTime> (_cooldowns, keyed {serverId}:{metricName}) checked against App.EmailCooldownMinutes. It is never persisted, so an app restart clears every cooldown and an alert that fired recently can immediately fire again.

UI acknowledgements (alert_state.json) do persist, but only clear on a genuinely newer event.

Suggested fix

Persist the cooldown state so it survives restarts. The config_alert_log table already records every alert with alert_time, server_id, metric_name — the cooldown check could read the most recent alert_time for the key instead of relying solely on the in-memory dictionary.

Component

Lite. Reported on 2.10.0.