feat(auth): implement session management for user authentication

- Add Session model to Prisma schema for session handling
- Implement session generation in `generateSession` utility
- Update registration and login APIs to create and set sessions
- Add middleware to skip user existence check in development
- Add Prisma client singleton for better connection management
- Include `@prisma/nuxt` module and `cookie` package in dependencies
- Create migration for session table and unique token index