Persist failed-job tray watermark across restart (both apps)

[erikdarlingdata]

Problem

The failed-Agent-job tray watermark (_lastAlertedFailedJobTime) was an in-memory dictionary in both apps and never persisted. Blocking/deadlock watermarks got restart persistence in #1145; the failed-job path never did. So on reopen the watermark was empty, every failure still inside the lookback window looked "new," and the toast re-fired for alerts the user had already seen and dismissed before the restart.

Fix (both apps, parity)

Persist the exact server-local newestFailure value and re-seed it on startup, mirroring #1145. The persisted value and the in-session compared value (FailedJobInfo.RunDateTime) always share a basis by construction — the UTC alert_time is never mixed in (the trap: RunDateTime is server-local, alert_time is host-UTC).

Lite (DuckDB): nullable watermark_time column on the existing config_edge_trigger_watermarks table (schema v30→v31 migration + fresh CREATE); Save/LoadFailedJobWatermarksAsync; seed in SeedEdgeTriggerWatermarksAsync, persist on fire. Count-watermark load filtered by watermark_time IS NULL so count and time rows stay cleanly separated.

Dashboard (JSON prefs): FailedJobAlertWatermarkTicks on UserPreferences (server-local time as DateTime.Ticks — basis-exact across JSON, no DateTimeKind drift); lazy-seed on first sweep, persist on fire via the existing SavePreferences pattern.

Result: after a reopen, already-alerted failures stay suppressed; only a genuinely new failure (including one that happened while the app was closed) alerts.

Tests

• Lite store round-trip: exact-value preservation, upsert, no bleed between count and time rows.
• Dashboard prefs round-trip + legacy-JSON-without-the-key.
• Lite 545/545, Dashboard 491/491, both apps build clean.

Live verification (recommended post-merge)

The persistence layer is unit-tested; the seed→suppress path lives in MainWindow. Behavioral check: fire a failed-job toast → dismiss → close + reopen → confirm it does not re-fire for that failure, while a new failure still does.

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