fix: eliminate SSE buffering through Nginx proxy

Add 2 KB padding comment after headers to push past proxy buffer
threshold, enable TCP_NODELAY on the socket, and remove erroneous
chunked_transfer_encoding off from Nginx that caused full response
buffering.

backend/src/routes/import.ts

@@ -32,6 +32,15 @@ function sseWrite(res: import('express').Response, data: Record<string, unknown>
   res.write(`data: ${JSON.stringify(data)}\n\n`);
 }
 
+/**
+ * Send a 2 KB comment block to push past any proxy buffering threshold.
+ * Nginx and other reverse proxies often buffer the first few KB before
+ * starting to stream — this padding forces the initial flush.
+ */
+function ssePadding(res: import('express').Response) {
+  res.write(`: ${' '.repeat(2048)}\n\n`);
+}
+
 const router = Router();
 
 router.post(
@@ -60,7 +69,9 @@ 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();
+      ssePadding(res);
 
       try {
         const converted = await convertPdfToStatementStreaming(

frontend/nginx.conf

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