build: Django 5.2.17 LTS

Django 4.2 reached end of life on 2026-04-07.

- Django==5.2.17; django-login-required-middleware 0.9.0 keeps working
  (the authorization matrix passes).
- USE_L10N removed from the settings template.
- Log out is a POST form in the navbar; Django 5 removed GET logout.
- pip-audit in CI no longer ignores the Django 4.2.30 advisories.
- README: Django 5.2, backup step and upgrade notes for existing
  settings and GET logout callers.

python -Wd manage.py test shows no Django deprecation warnings. The live
libvirt suite passes on 5.2 and on 4.2 against a RHEL qemu-kvm host.

templates/navbar.html

@@ -44,7 +44,10 @@
             </a>
             <a class="dropdown-item {% view_active request 'accounts:profile' %}" href="{% url 'accounts:profile' %}">{% bs_icon 'person-vcard' %} {% trans "Profile" %}</a>
             <div class="dropdown-divider"></div>
-            <a class="dropdown-item" href="{% url 'accounts:logout' %}">{% bs_icon 'power' %}  {% trans "Log Out" %}</a>
+            <form method="post" action="{% url 'accounts:logout' %}">
+              {% csrf_token %}
+              <button type="submit" class="dropdown-item">{% bs_icon 'power' %}  {% trans "Log Out" %}</button>
+            </form>
           </div>
         </li>
       </ul>