fix(ci): use SCP to copy docker-compose and nginx from repo instead of hardcoded cat

- Add proxy to build matrix
- Replace heredoc docker-compose generation with appleboy/scp-action
- Fix nginx upstream backend port to 3000 (matches prod WEBSERVER_PORT)
- Copy docker-compose.yml and nginx.conf from repo via SCP staging

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

infra/docker/nginx/nginx.conf

@@ -1,5 +1,5 @@
 upstream backend {
-    server backend:3001;
+    server backend:3000;
 }
 
 upstream frontend {