feat(admin): bulk-editable article list with WP ID inline editing

- New /admin/article-list: paginated (50/page) table with thumbnail,
  title, excerpt (120 chars), status, scheduled date, and WP ID input
- Sticky save bar with live change counter (JS tracks modified inputs,
  highlights changed cells in amber, disables save when nothing changed)
- POST /admin/article-list/update: saves only changed WP IDs in one
  request; clears stale wp_post_url so WP-Sync repopulates it cleanly
- Filter by status + free-text search (title or article ID)
- Pagination with page/filter state preserved through save redirects
- repositories: add list_articles_page() (offset + search) and
  bulk_update_wp_post_ids()
- Dashboard nav: add Artikelliste link

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/templates/admin_dashboard.html

@@ -13,6 +13,7 @@
       <p>Angemeldet als <strong>{{ user }}</strong></p>
     </div>
     <div class="row">
+      <a class="linkbtn" href="/admin/article-list">Artikelliste</a>
       <a class="linkbtn" href="/admin/schedule">Veröffentlichungsplan</a>
       <a class="linkbtn" href="/admin/connectivity">Connectivity Check</a>
       <form method="post" action="/admin/logout">