Pin spaces==0.50.4 to restore fast ZeroGPU inference

spaces 0.51.0 (released 2026-07-10) changed tensor packing behavior,
causing inference to jump from 3-8s to 22-75s after the Space restarted
post-AWS outage on 2026-07-17. Pinning to the version that was working.

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

requirements.txt

@@ -7,7 +7,7 @@ pyarrow
 sentencepiece
 torchvision
 kernels
-spaces
+spaces==0.50.4
 hf_xet
 gradio
 pytest