feat(updates): "What's New" highlights after update + pre-update teaser

Surfaces release highlights to admins, sourced from the GitHub release notes
(no AI at runtime). Bullets are written once per release in CI via GitHub Models
(see docs/ci/whatsnew-highlights.yml) into a <!-- whatsnew --> block; the app
reads that block and falls back to the changelog's "### Features" for releases
without it — so it works against today's releases immediately.

- backend utils/whatsNew.parseWhatsNew(body): curated block else Features
  section, strips scope/PR-links, de-dups, caps at 8 (tested).
- GET  /admin/system/updates/whatsnew: highlights for every version moved
  through since the per-instance marker (whatsnew_last_seen_version); fresh
  installs self-anchor silently. Best-effort, never errors.
- POST /admin/system/updates/whatsnew/seen: advance the marker (per-instance).
- /admin/system/updates also returns latestHighlights for the teaser.
- Frontend: WhatsNewBanner (green bar -> modal with "Full changelog" link) on
  the dashboard via adminService; UpdateNotification shows a "New features
  include:" teaser. i18n de/en. No migration (uses app_settings).

frontend/src/pages/admin/AdminDashboard.tsx

@@ -22,6 +22,7 @@ import { useLocalizedDate } from '../../hooks/useLocalizedDate';
 
 import { Button, Card, Loading } from '../../components/common';
 import { UpdateNotification } from '../../components/admin/UpdateNotification';
+import { WhatsNewBanner } from '../../components/admin/WhatsNewBanner';
 import { CrmOverviewSection } from '../../components/admin/CrmOverviewSection';
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
 import { eventsService } from '../../services/events.service';
@@ -184,6 +185,8 @@ export const AdminDashboard: React.FC = () => {
 
   return (
     <div>
+      {/* After-update "What's New" highlights (above the update banner) */}
+      <WhatsNewBanner />
       {/* Update Notification */}
       <UpdateNotification />