Add cache-busting query strings and no-cache for HTML files

Proxy was caching old admin.js; ?v=3 query string forces a fresh fetch.
Also extend no-cache header to .html files so future deploys propagate immediately.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/admin.html

@@ -793,6 +793,6 @@
     </div>
   </div>
 
-  <script src="admin.js"></script>
+  <script src="admin.js?v=3"></script>
 </body>
 </html>