Revert SSE streaming for PDF import, use synchronous flow

SSE streaming added unnecessary complexity and latency due to
buffering issues across Node.js event loop, Nginx proxy, and
Docker layers. Reverted to a simple synchronous request/response
for PDF conversion. Kept extractLlmErrorMessage for user-friendly
LLM errors, lazy-loaded pdf-parse, and extended Nginx timeout.

frontend/nginx.conf

@@ -3,7 +3,7 @@ server {
     root /usr/share/nginx/html;
     index index.html;
 
-    # Import endpoint — SSE streaming, long timeout, no buffering
+    # Import endpoint — long timeout for LLM processing
     location /api/import {
         proxy_pass http://family-budget-backend:3000;
         proxy_http_version 1.1;
@@ -14,8 +14,6 @@ server {
         proxy_cookie_path / /;
         proxy_connect_timeout 5s;
         proxy_read_timeout 600s;
-        proxy_buffering off;
-        gzip off;
         client_max_body_size 15m;
     }