Replace torchaudio.save with soundfile.write; add EPUB loader node

- nodes/generator.py: swap torchaudio.save for soundfile.write to avoid
  torchcodec/FFmpeg dependency crash in environments without FFmpeg shared libs
- nodes/epub_loader.py: new OmniVoiceEpubLoader node for loading EPUB chapters
- tests/test_epub_loader.py: 8 tests for the EPUB loader
- install.py: add beautifulsoup4 to runtime deps
- __init__.py, nodes/__init__.py: register OmniVoiceEpubLoader

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

requirements.txt

@@ -1,3 +1,4 @@
 # Dependencies are managed by install.py to avoid overwriting ComfyUI's torch.
 # omnivoice pins torch==2.8.* (CUDA 12.8) which would break ComfyUI's torch build.
 # Do not add omnivoice here — install.py handles it with --no-deps.
+# EPUB parsing (OmniVoiceEpubLoader) requires beautifulsoup4 (installed by install.py).