Add maintenance page served by nginx during deploys

When the SvelteKit upstream is unreachable (container restart, deploy)
nginx now serves a static, brand-styled "Be right back" page instead
of the default 502/503. Auto-reloads after 60s so visitors don't sit
on it once the app is back.

- nginx/maintenance.html: self-contained, no external assets, inline
  paw SVG, brand colours, contact details fallback
- nginx/nginx.conf: proxy_intercept_errors + error_page 502/503/504
  on both location blocks; 2s proxy_connect_timeout so nginx fails
  over fast instead of holding the connection for 60s

Deploy note: the html file needs to live at /var/www/html/maintenance.html
inside the nginx container (already mounted from /docker/wordpress/goodwalk.co.nz/html).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

nginx/nginx.conf

@@ -29,6 +29,14 @@ server {
   gzip on;
   gzip_types text/plain text/css application/javascript application/json image/svg+xml;
 
+  # Served when the SvelteKit upstream is unreachable (deploys, restarts).
+  # Mounted into the nginx container at /var/www/html/maintenance.html.
+  location = /maintenance.html {
+    root /var/www/html;
+    internal;
+    add_header Cache-Control "no-store" always;
+  }
+
   location /api/submit {
     proxy_pass http://mail-api:8000/submit;
     proxy_http_version 1.1;
@@ -36,6 +44,10 @@ server {
     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_intercept_errors on;
+    proxy_connect_timeout 2s;
+    error_page 502 503 504 = /maintenance.html;
   }
 
   location / {
@@ -47,5 +59,13 @@ server {
     proxy_set_header X-Forwarded-Proto $scheme;
     proxy_set_header Upgrade $http_upgrade;
     proxy_set_header Connection "upgrade";
+
+    # Serve a static maintenance page when the app container is down/restarting,
+    # rather than nginx's default 502/503. The `=` keeps the original status
+    # code so search engines treat it as a transient outage, not a real page.
+    proxy_intercept_errors on;
+    proxy_connect_timeout 2s;
+    proxy_next_upstream error timeout http_502 http_503 http_504;
+    error_page 502 503 504 = /maintenance.html;
   }
 }