Persist the Lite alert email cooldown across restarts (#981)

[erikdarlingdata]

Problem

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

EmailAlertService deduplicated alert emails with an in-memory ConcurrentDictionary<string, DateTime> cooldown keyed {serverId}:{metricName}. It was never persisted, so restarting Lite cleared every cooldown — an alert email sent minutes before the restart could be sent again immediately after.

Change

The first time each alert key is evaluated, the cooldown is seeded from config_alert_log via the new GetLastEmailSentUtcAsync — the most recent alert_time for that server/metric where an email actually succeeded (notification_type IN ('email','email+webhook') AND send_error IS NULL, which is exactly when _cooldowns is updated after SendEmailAsync). The in-memory dictionary stays authoritative once seeded, so it's one small read per key per app session.

Test plan

• dotnet build Lite/PerformanceMonitorLite.csproj -c Debug — full rebuild, 0 warnings, 0 errors.
• The cooldown query was run against DuckDB with representative config_alert_log rows: it correctly returns the most recent successful email send, excluding a later row with a send_error (failed send) and a tray-only row, and returns null when there's no history for the key.
• Column names verified against the config_alert_log INSERT in LogAlertAsync.

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