fix: use correct block name in messages template + add TPL-008 rule

- Fix messages.html: change {% block page_scripts %} to {% block extra_scripts %}
  (page_scripts doesn't exist in admin/base.html, causing JS not to load)

- Add TPL-008 architecture rule to catch invalid template block names
  This prevents silent failures where content in undefined blocks is ignored

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/templates/admin/messages.html

@@ -331,6 +331,6 @@
 {% endcall %}
 {% endblock %}
 
-{% block page_scripts %}
+{% block extra_scripts %}
 <script src="{{ url_for('static', path='admin/js/messages.js') }}"></script>
 {% endblock %}