libretranslate: pin uv python to 3.12 (pytorch fix)

[MickLesk]

✍️ Description

PyTorch versions below 2.5.0 don't provide wheels for Python 3.13 (cp313). Debian 13 uses Python 3.13 by default, causing the installation to fail with 'no wheels with a matching Python ABI tag'.

Use uv venv --python 3.12 to create the virtual environment with Python 3.12, ensuring compatibility with all PyTorch versions.

🔗 Related PR / Issue

Link: #9696

✅ Prerequisites (X in brackets)

• Self-review completed – Code follows project standards.
• Tested thoroughly – Changes work as expected.
• No security risks – No hardcoded secrets, unnecessary privilege escalations, or permission issues.

---

🛠️ Type of Change (X in brackets)

• 🐞 Bug fix – Resolves an issue without breaking functionality.
• ✨ New feature – Adds new, non-breaking functionality.
• 💥 Breaking change – Alters existing functionality in a way that may require updates.
• 🆕 New script – A fully functional and tested script or script set.
• 🌍 Website update – Changes to website-related JSON files or metadata.
• 🔧 Refactoring / Code Cleanup – Improves readability or maintainability without changing functionality.
• 📝 Documentation update – Changes to README, AppName.md, CONTRIBUTING.md, or other docs.