protocol Repository  :4-7
  func findById(_ id) → User?  :5-5
  func save(_ user)  :6-6
class UserService  :9-25
  func getUser(id) → User?  :16-16
  async func createUser(data) → User  :20-20
struct User  :27-30
enum UserRole  :32-34
func hashPassword(_ password) → String  :36-38
