Pin diffusers to 0.39.0 release to fix hub dependency conflict

The Space build failed: diffusers' git HEAD (0.40.0.dev0) recently
raised its huggingface-hub floor to >=1.23.0, which has zero overlap
with transformers==4.57.1's huggingface-hub<1.0 ceiling — pip's
resolver had no valid huggingface-hub version to pick. This was a
latent conflict from tracking diffusers' unpinned main branch, not
something introduced by the last commit's bitsandbytes addition.

Verified against a real pip resolve that every diffusers symbol this
app actually imports (AttentionMixin, attention_dispatch, CacheMixin,
AutoencoderKLQwenImage, etc. — the app vendors its own pipeline/
transformer classes locally) exists in 0.39.0, and that pinning to
0.39.0 resolves cleanly alongside transformers==4.57.1, bitsandbytes,
and accelerate/peft still on git HEAD. Preferred over bumping
transformers to its new 5.x line, which would risk breaking API
changes with no way to test end-to-end here.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

requirements.txt

@@ -1,5 +1,5 @@
 git+https://github.com/huggingface/accelerate.git
-git+https://github.com/huggingface/diffusers.git
+diffusers==0.39.0
 git+https://github.com/huggingface/peft.git
 transformers==4.57.1
 bitsandbytes