feat(deploy): template the nginx vhosts with the deployment domain

ikerdemo.cat was hardcoded in six server blocks, so the stack could only be
deployed under that one domain. The vhosts are now a template rendered by the
nginx entrypoint from MUSIFY_ROOT_DOMAIN, with the envsubst filter set so it
does not eat nginx's own variables. The repeated proxy headers move to a
shared snippet, the web player vhost gets the upload body limit it was missing
(audio is posted to its form actions), and the streaming vhost turns off proxy
buffering so range requests stay responsive.

deploy/nginx/snippets/proxy.conf

@@ -0,0 +1,9 @@
+# Shared proxy settings for every Musify vhost.
+proxy_http_version 1.1;
+proxy_set_header Host              $host;
+proxy_set_header X-Real-IP         $remote_addr;
+proxy_set_header X-Forwarded-For   $proxy_add_x_forwarded_for;
+proxy_set_header X-Forwarded-Proto https;
+proxy_set_header X-Forwarded-Host  $host;
+proxy_set_header Upgrade           $http_upgrade;
+proxy_set_header Connection        $connection_upgrade;