feat(admin): add restore dismissed alerts flow

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

tests/server/api/admin-auth-guards.test.js

@@ -42,7 +42,9 @@ const adminRoutes = {
   ],
   'admin/alerts/': [
     'alerts/index.get.js',
-    'alerts/dismiss.post.js'
+    'alerts/dismiss.post.js',
+    'alerts/dismissed.get.js',
+    'alerts/restore.post.js'
   ]
 }
 
@@ -68,7 +70,9 @@ const businessLogicPatterns = [
   'PreRegistration.aggregate',
   'PreRegistration.updateMany',
   'computeAllAlerts(',
-  'AdminAlertDismissal.findOneAndUpdate'
+  'AdminAlertDismissal.findOneAndUpdate',
+  'AdminAlertDismissal.find',
+  'AdminAlertDismissal.deleteMany'
 ]
 
 describe('Admin endpoint auth guards', () => {