fix sse prefix, after switching from nginx proxy to ALB (#2842)
* fix sse prefix, after switching from nginx proxy to ALB
same issue as for /admin, which was fixed in three PR:
https://github.com/huggingface/dataset-viewer/pull/2820,
https://github.com/huggingface/dataset-viewer/pull/2821,
https://github.com/huggingface/dataset-viewer/pull/2825
* fix test