Fix monitor timeline no-data states

[simlarsen]

Summary

• render days with no monitoring coverage as neutral gray instead of healthy green, using half-open millisecond-precise day boundaries
• persist no-data gaps when monitors are directly disabled and safely resume their current status when re-enabled
• serialize bulk/single monitor transitions and recover failed hooks, late status writes, rapid toggles, and rolling-deploy writes from older pods
• repair existing timeline drift with a restartable data migration plus a bounded startup/hourly safety reconciler

Root cause

The Dashboard passed healthy green as its empty-day fallback, while disabling a monitor left the latest status timeline row open. Open rows are extended to the present, so disabled or otherwise unobserved time appeared healthy.

Testing

• npm run fix
• cd Common && npm run compile
• cd App && npm run compile
• cd App/FeatureSet/Dashboard && npm run compile
• 91 new regression tests covering day classification, half-open and exact-ms boundaries, pause/resume lifecycle, per-monitor locking, retries, failure recovery, old-pod races, late status writes, orphan protection, migration repair, pagination, and bounded recurring reconciliation
• 120 relevant checks passed (114 Common + 6 App)

Fixes #576

[simlarsen]

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