Document audiocraft incompatibility with Python 3.13

audiocraft (MusicGen) is NOT compatible with Python 3.13 due to:
- audiocraft → spacy → thinc → blis
- blis fails to compile with GCC errors: unrecognized '-mavx512pf' option
- This is a known issue with blis and newer GCC/Python versions

Updated requirements.txt to:
- Remove audiocraft from direct dependencies
- Add note about Python 3.13 incompatibility
- Suggest using Python 3.12 or lower for audiocraft
- Or use a separate Python 3.12 environment for music generation