Page is sent with Cache-Control: no-cache (closes #141)

nginx sent index.html with no Cache-Control, so a browser could keep
it after a deploy and ask for hashed assets the new build no longer
has, and show an empty page. location / now adds Cache-Control:
no-cache, which covers its try_files fallback to /index.html, and
includes security-headers.conf again, since its own add_header drops
the server's. /assets/ and the locations passed to the backend are
unchanged.

script/frontend-viewport-test fails unless / and a path that falls
back to the page carry Cache-Control: no-cache and
Content-Security-Policy, which stands for the security headers.

Model: opus-5-5

configs/nginx.conf

@@ -30,8 +30,12 @@ server {
     access_log /dev/stdout combined;
     error_log  /dev/stderr warn;
 
+    # The page is revalidated on every load, so a browser never keeps a
+    # copy that names assets a newer build no longer has.
     location / {
         try_files $uri $uri/ /index.html;
+        add_header Cache-Control no-cache;
+        include /etc/nginx/security-headers.conf;
     }
 
     # Cache static assets aggressively