feat: make next-auth peer dependency optional

[conarti]

🔗 Linked issue

#1071

❓ Type of change

• 📖 Documentation (updates to the documentation, readme or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Added peerDependenciesMeta to make next-auth an optional peer dependency.

This prevents automatic installation of next-auth (~50MB including Next.js) for users who only use the local provider.

Before: next-auth is always installed automatically (npm v7+ behavior)
After: next-auth is only installed when explicitly added by users who need the authjs provider

📝 Checklist

• I have linked an issue or discussion.
• I have added tests (if possible).
• I have updated the documentation accordingly.