Add torchaudio via PyTorch cu121 extra-index-url in requirements.txt

pre-install.sh is not auto-executed by HF Spaces. Using
--extra-index-url in requirements.txt ensures pip fetches the
CUDA 12.1-compatible torchaudio wheel that matches the pre-installed
torch on the ZeroGPU worker.

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

requirements.txt

@@ -16,3 +16,5 @@ spaces==0.48.1
 gradio==5.25.0
 pydantic==2.8.2
 av
+--extra-index-url https://download.pytorch.org/whl/cu121
+torchaudio