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.

backend/src/routes/import.ts

@@ -28,8 +28,14 @@ function isJsonFile(file: { mimetype: string; originalname: string }): boolean {
   );
 }
 
+const SSE_PAD_TARGET = 4096;
+
 function sseWrite(res: import('express').Response, data: Record<string, unknown>) {
-  res.write(`data: ${JSON.stringify(data)}\n\n`);
+  const payload = `data: ${JSON.stringify(data)}\n\n`;
+  const pad = Math.max(0, SSE_PAD_TARGET - payload.length);
+  // SSE comment lines (": ...") are ignored by the browser but push
+  // data past proxy buffer thresholds so each event is delivered immediately.
+  res.write(pad > 0 ? `: ${' '.repeat(pad)}\n${payload}` : payload);
 }
 
 const router = Router();
@@ -60,6 +66,7 @@ router.post(
       res.setHeader('Cache-Control', 'no-cache');
       res.setHeader('Connection', 'keep-alive');
       res.setHeader('X-Accel-Buffering', 'no');
+      res.socket?.setNoDelay(true);
       res.flushHeaders();
 
       try {

frontend/nginx.conf

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