Consolidate gateway-gamers sites, dedupe shared header/footer/images

Merge the four independent gateway-gamers.net site repos (40k-rankings,
aos-rankings, kingmaker, scouting) into one monorepo under sites/,
preserving each site's full commit history via a manual subtree-style
merge (git-subtree isn't available in this environment).

- shared/: header.template.html (nav swapped in per site), footer.html,
  and images/ — previously byte-identical copies duplicated 4x (~100MB).
- Each site's public/footer.html and public/images are now symlinks into
  shared/; public/header.html is generated at predev/prebuild time from
  shared/header.template.html + the site's own nav.html, so nav bars stay
  individual while everything else about the header is shared.
- Single root .gitlab-ci.yml with path-scoped build/deploy jobs per site
  plus a deploy:shared job, replacing the 4 separate per-repo pipelines.
- nginx confs (kept per-site as reference, same as before) gain
  location /images/ and location = /footer.html alias blocks pointing at
  a shared webserver directory, so the deployed site no longer duplicates
  the images either — see README.md for the full layout and the runbook
  for applying this on the webserver.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SBzcNkW7JcYipnAX6HfgmK

sites/40k-rankings/40k-rankings.gateway-gamers.net.conf

@@ -30,6 +30,16 @@ server {
     add_header Permissions-Policy "geolocation=(), microphone=(), camera=()" always;
     add_header Content-Security-Policy "default-src 'self'; script-src 'self'; style-src 'self'; connect-src 'self' https://newprod-api.bestcoastpairings.com; img-src 'self' data:;" always;
 
+    # Shared assets (images + footer) — single on-disk copy under shared/,
+    # deployed once and aliased by every gateway-gamers.net site.
+    location /images/ {
+        alias /var/www/domains/gateway-gamers.net/shared/images/;
+    }
+
+    location = /footer.html {
+        alias /var/www/domains/gateway-gamers.net/shared/footer.html;
+    }
+
     location / {
         try_files $uri $uri/ =404;
     }