docs: add nginx tenant onboarding template and backend handoff doc

Adds a copy-paste server block template for onboarding a new marketplace
domain, and a Sprint 16 backend handoff doc covering the still-missing
draft/publish persistence endpoints, server-side validation expectations,
and the slug/route inconsistency in static-page data.

nginx.conf

@@ -94,3 +94,57 @@ server {
     add_header X-XSS-Protection "1; mode=block" always;
     add_header Referrer-Policy "strict-origin-when-cross-origin" always;
 }
+
+# Template for onboarding a new marketplace tenant.
+# Replace NEWMARKETPLACE.EXAMPLE.COM, /var/www/newmarketplace, and the
+# api.newmarketplace.example.com:443 proxy target with the real values,
+# then rename this block's server_name/root before deploying.
+server {
+    listen 80;
+    server_name newmarketplace.example.com www.newmarketplace.example.com;
+
+    root /var/www/newmarketplace/browser;
+    index index.html;
+
+    # Angular routing - serve index.html for all routes (client-side router
+    # handles /edit, /:lang/edit/:section, etc. once index.html is served)
+    location / {
+        try_files $uri $uri/ /index.html =404;
+    }
+
+    # Proxy API calls to backend
+    location /api {
+        proxy_pass https://api.newmarketplace.example.com:443;
+        proxy_set_header Host api.newmarketplace.example.com;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        rewrite ^/api(/.*)$ $1 break;
+        proxy_ssl_verify off;
+    }
+
+    # Static assets caching
+    location ~* \.(js|css|png|jpg|jpeg|gif|ico|svg|woff|woff2|ttf|eot)$ {
+        expires 1y;
+        add_header Cache-Control "public, immutable";
+        try_files $uri =404;
+    }
+
+    # Don't cache index.html
+    location = /index.html {
+        add_header Cache-Control "no-cache, no-store, must-revalidate";
+        add_header Pragma "no-cache";
+        add_header Expires "0";
+    }
+
+    gzip on;
+    gzip_vary on;
+    gzip_proxied any;
+    gzip_comp_level 6;
+    gzip_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript image/svg+xml;
+    gzip_min_length 1000;
+
+    add_header X-Frame-Options "SAMEORIGIN" always;
+    add_header X-Content-Type-Options "nosniff" always;
+    add_header X-XSS-Protection "1; mode=block" always;
+    add_header Referrer-Policy "strict-origin-when-cross-origin" always;
+}