fix(api): derive host from storefront domain

Every storefront, including nested subdomains, uses its matching api.<hostname> endpoint.

scripts/deploy/server-setup.sh

@@ -104,18 +104,6 @@ server {
         proxy_read_timeout 60s;
     }
 
-    # One same-origin gateway for bootstrap, auth, legacy and versioned APIs.
-    # The trailing slash removes /backend/ before forwarding upstream.
-    location /backend/ {
-        proxy_pass http://127.0.0.1:8080/;
-        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 $scheme;
-        proxy_read_timeout 60s;
-    }
-
     # SPA fallback. Must stay last: every unmatched path is a client route.
     location / {
         try_files $uri $uri/ /index.html;