feat: add admin video jobs management endpoints and UI for listing, cancelling, and purging video jobs

Co-authored-by: Copilot <copilot@github.com>

frontend/app/templates/base.html

@@ -30,6 +30,7 @@
           <a href="{{ url_for('profile') }}">Profile</a>
           {% if session.get('user_role') == 'admin' %}
           <a href="{{ url_for('admin') }}">Admin</a>
+          <a href="{{ url_for('admin_videos') }}">Video Jobs</a>
           {% endif %}
           <a href="{{ url_for('logout') }}">Log out</a>
           {% else %}