- Add User struct with username/email/password/first_name/last_name/role/is_active/last_login fields and GORM tags
- Add Role struct with name/description/permissions many2many relationship and GORM tags
- Add Permission struct with name/resource/action/description fields for RBAC
- Add AuthSession struct with user_id/token/refresh_token/expires_at/ip_address/user_agent/is_active fields
- Add APIKey struct with name