feat(gateway): add nginx gateway for maintenance mode fallback

Proxy frontend traffic through Nginx to handle service restarts gracefully
Serve a static 503 maintenance page when frontend or backend is unavailable
Update deployment design docs and docker-compose configuration

frontend/gateway/nginx.conf

@@ -0,0 +1,45 @@
+server {
+  listen 20015;
+  server_name _;
+
+  resolver 127.0.0.11 valid=5s ipv6=off;
+
+  location / {
+    auth_request /backend-health;
+    error_page 500 502 503 504 =503 /maintenance.html;
+
+    set $frontend_upstream http://frontend:20015;
+
+    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_connect_timeout 1s;
+    proxy_send_timeout 30s;
+    proxy_read_timeout 30s;
+    proxy_intercept_errors on;
+
+    proxy_pass $frontend_upstream;
+  }
+
+  location = /backend-health {
+    internal;
+    set $backend_upstream http://backend:3001/health;
+
+    proxy_pass_request_body off;
+    proxy_set_header Content-Length "";
+    proxy_connect_timeout 1s;
+    proxy_read_timeout 1s;
+
+    proxy_pass $backend_upstream;
+  }
+
+  location = /maintenance.html {
+    internal;
+    root /usr/share/nginx/html;
+    add_header Cache-Control "no-store" always;
+    add_header Retry-After "300" always;
+  }
+}