fix: production CSRF, ProxyFix, and SSE streaming issues

nginx/nginx.prod.conf

@@ -67,6 +67,22 @@ server {
     gzip_types text/plain text/css application/json application/javascript text/xml application/xml text/javascript image/svg+xml;
     gzip_min_length 1000;
 
+    # SSE streaming for assistant chat
+    location /api/assistant/chat/stream {
+        proxy_pass http://backend;
+        proxy_set_header Host $host;
+        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;
+        proxy_set_header Connection '';
+        proxy_http_version 1.1;
+        proxy_buffering off;
+        proxy_cache off;
+        chunked_transfer_encoding off;
+        proxy_read_timeout 300s;
+        proxy_send_timeout 300s;
+    }
+
     # API requests → Flask
     location /api/ {
         proxy_pass http://backend;