Fix frontend nginx to use Railway PORT variable

- Use envsubst to inject PORT at runtime
- Template nginx.conf with ${PORT} placeholder

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/nginx.conf

@@ -1,5 +1,5 @@
 server {
-    listen 80;
+    listen ${PORT};
     server_name _;
     root /usr/share/nginx/html;
     index index.html;