Admins can now surface dismissed alert types without waiting for the
underlying data to change. Adds a collapsible "Restore dismissed"
section below the active alerts with per-type checkboxes.
- ALERT_METADATA map in adminAlerts.js as the single source of truth
for slug → title/severity; detectors refactored to reference it
- GET /api/admin/alerts/dismissed returns this admin's dismissals
joined with metadata (title, severity, dismissedAt)
- POST /api/admin/alerts/restore deletes dismissals by alertType[],
returns the deleted count
- AdminAlertsPanel fetches both active + dismissed; stays visible
when either is non-empty; checkboxes + "Restore selected" button
- adminAlertRestoreSchema validates the POST body against the enum
- Auth guards test covers both new routes
