Add user authentication infrastructure with login/logout functionality,
session management, and basic user model.
Changes:
- Create users table with email, password_digest, confirmation tokens
- Add Users repository with authentication methods
- Add User entity with authentication helpers
- Implement login form and authentication actions
- Add logout action
- Add BCrypt gem for password hashing
- Update base Action class with authentication helpers
- Add login/logout routes
Next steps: Registration flow, password reset, email confirmation
david