fix(nginx): exclude /api/ paths from static asset location block

The static asset regex matched .png in /api/v1/og/contributions.png
before the /api/ proxy block, returning 404. Add negative lookahead
to skip /api/ paths.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

asset/nginx/site.conf.tmpl

@@ -20,7 +20,7 @@ server {
         add_header Cache-Control "no-cache" always;
     }
 
-    location ~* \.(js|css|woff2?|ttf|eot|svg|png|jpg|jpeg|gif|ico|webp|avif)$ {
+    location ~* ^(?!/api/)\S+\.(js|css|woff2?|ttf|eot|svg|png|jpg|jpeg|gif|ico|webp|avif)$ {
         expires 30d;
         add_header Cache-Control "public, max-age=2592000, immutable";
         try_files $uri =404;