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/test-auth-flow.html

@@ -217,7 +217,7 @@
 </div>
 {% endblock %}
 
-{% block scripts %}
+{% block extra_scripts %}
 <script>
 function authFlowTest() {
     return {