fix(kong): disable request buffering for large file uploads

Add KONG_NGINX_PROXY_PROXY_REQUEST_BUFFERING=off so Kong streams
the request body to admin-service without buffering to disk.

Root cause: Nginx streams (proxy_request_buffering off) → Kong buffers
to disk → Kong returns 400 with empty body when forwarding to upstream.
Fix: Kong also streams, matching Nginx's streaming behavior end-to-end.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/docker-compose.yml

@@ -182,6 +182,7 @@ services:
       KONG_ADMIN_ERROR_LOG: /dev/stderr
       KONG_ADMIN_LISTEN: 0.0.0.0:8001
       KONG_PROXY_LISTEN: 0.0.0.0:8080
+      KONG_NGINX_PROXY_PROXY_REQUEST_BUFFERING: "off"
     ports:
       - "48080:8080"    # Proxy (frontend connects here)
       - "127.0.0.1:48001:8001"    # Admin API