CodiMD: allow cors for origin stuebinm.4future.dev (a:f cluster site)

Allow cors, so that the about:future cluster site can fetch its content from a codimd pad
(which is currently a preview at https://stuebinm.4future.dev/about-future-website/ but will
be moved to a better position once there's a domain for that).

configuration/hosts/hainich/services/codimd.nix

@@ -72,6 +72,7 @@
         proxy_set_header X-Forwarded-Host $http_host;
         proxy_set_header Upgrade $http_upgrade;
         proxy_set_header Connection $http_connection;
+        proxy_add_header Access-Control-Allow-Origin "https://stuebinm.4future.de";
         proxy_buffering off;
       '';
     };