fix(nginx): strip /api/ prefix when proxying to backend

Add trailing slash to proxy_pass directive so nginx strips the /api/
prefix before forwarding requests to the API container. This fixes
routes like /api/docs being passed as /api/docs instead of /docs.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

frontend/nginx.conf.template

@@ -15,7 +15,7 @@ server {
 
     # Proxy API requests to backend
     location /api/ {
-        proxy_pass ${API_URL};
+        proxy_pass ${API_URL}/;
         proxy_http_version 1.1;
         proxy_set_header Upgrade $http_upgrade;
         proxy_set_header Connection 'upgrade';