fix: increase Nginx proxy timeout to 300s for long-running API calls

MITRE sync and data source imports can take over 60s on first run,
causing Nginx to return 504 Gateway Timeout to the frontend.

frontend/nginx.conf

@@ -26,6 +26,11 @@ server {
         proxy_set_header X-Real-IP $remote_addr;
         proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
         proxy_set_header X-Forwarded-Proto $scheme;
+
+        # Long-running operations (MITRE sync, data source imports) need more time
+        proxy_read_timeout 300s;
+        proxy_connect_timeout 10s;
+        proxy_send_timeout 300s;
     }
 
     # Health endpoint proxy