Disable CDN caching for HTML, add Quicklink prefetching

- Change Cache-Control from s-maxage=31536000 to max-age=0,must-revalidate
  for all HTML pages and sitemap (fixes stale content after settings change)
- Remove Cloudflare PurgeURLs call since we're not caching HTML at CDN
- Add Quicklink for client-side prefetching on public blog pages
- Make main branch deploy automatic (remove manual trigger)

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

internal/build/templates/base.html

@@ -71,6 +71,8 @@
   </div>
 
   <script src="/static/js/main.js"></script>
+  <script src="https://unpkg.com/quicklink@2.3.0/dist/quicklink.umd.js"></script>
+  <script>quicklink.listen({ignores: [/\/studio/, /\/api\//]});</script>
   {{block "scripts" .}}{{end}}
 </body>
 </html>