fix: add TPL-008 check to main validation + fix 4 templates

The TPL-008 rule was only in the single-file validation path,
not in the full project validation. Added it to _validate_templates().

Fixed invalid block names:
- customers.html: page_scripts → extra_scripts
- notifications.html: page_scripts → extra_scripts
- test-vendors-users-migration.html: scripts → extra_scripts
- test-auth-flow.html: scripts → extra_scripts

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

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

app/templates/admin/notifications.html

@@ -356,6 +356,6 @@
 </div>
 {% endblock %}
 
-{% block page_scripts %}
+{% block extra_scripts %}
 <script src="{{ url_for('static', path='admin/js/notifications.js') }}"></script>
 {% endblock %}