app_held / app_hold_cleared allow-listed and operator-only; handleEvent mirrors them in held_apps; HeldAppsChecker
sends ONE app_held_reminder (warning, operator-only, per-app cooldown) after the grace per reason (6 h / 1 d / 3 d),
then weekly; recorded before sending; stale rows not reminded; the hub never clears a hold. Test-only
alerting.held_app_grace_override. TestD53_* in api/monitor/notify/web, red-proved by six mutations. Commit only.
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