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/api/admin/alerts/dismissed.get.js

@@ -0,0 +1,23 @@
+import AdminAlertDismissal from '../../../models/adminAlertDismissal.js'
+import { ALERT_METADATA } from '../../../utils/adminAlerts.js'
+import { connectDB } from '../../../utils/mongoose.js'
+
+export default defineEventHandler(async (event) => {
+  const admin = await requireAdmin(event)
+  await connectDB()
+
+  const rows = await AdminAlertDismissal.find({ adminId: admin._id.toString() })
+    .sort({ dismissedAt: -1 })
+    .lean()
+
+  const dismissed = rows
+    .filter((r) => ALERT_METADATA[r.alertType]) // defensive: skip any orphaned legacy slugs
+    .map((r) => ({
+      alertType: r.alertType,
+      title: ALERT_METADATA[r.alertType].title,
+      severity: ALERT_METADATA[r.alertType].severity,
+      dismissedAt: r.dismissedAt
+    }))
+
+  return { dismissed }
+})

server/api/admin/alerts/restore.post.js

@@ -0,0 +1,15 @@
+import AdminAlertDismissal from '../../../models/adminAlertDismissal.js'
+import { connectDB } from '../../../utils/mongoose.js'
+
+export default defineEventHandler(async (event) => {
+  const admin = await requireAdmin(event)
+  const { alertTypes } = await validateBody(event, adminAlertRestoreSchema)
+  await connectDB()
+
+  const result = await AdminAlertDismissal.deleteMany({
+    adminId: admin._id.toString(),
+    alertType: { $in: alertTypes }
+  })
+
+  return { ok: true, restored: result.deletedCount ?? 0 }
+})