fix: disable gzip and pad SSE events to prevent proxy buffering

Add gzip off to Nginx import location — the global gzip on was
buffering text/event-stream responses. Pad each SSE event to 4 KB
with comment lines to push past any remaining proxy buffer threshold.

frontend/nginx.conf

@@ -15,6 +15,7 @@ server {
         proxy_connect_timeout 5s;
         proxy_read_timeout 600s;
         proxy_buffering off;
+        gzip off;
         client_max_body_size 15m;
     }