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

server/utils/schemas.js

@@ -410,3 +410,10 @@ export const adminAlertDismissSchema = z.object({
   alertType: z.enum(ADMIN_ALERT_TYPES),
   signature: z.string().min(1).max(128)
 })
+
+export const adminAlertRestoreSchema = z.object({
+  alertTypes: z
+    .array(z.enum(ADMIN_ALERT_TYPES))
+    .min(1)
+    .max(ADMIN_ALERT_TYPES.length)
+})