func (u *UserRepository) ByUsername(username string) (*User, error) {
	if user, ok := u.Users[username]; ok {
		return user, nil
	}
	return nil, ErrNotFound
}
func (u *UserRepository) ByUsername(username string) (*User, error) {
	if user, ok := u.Users[username]; ok {
		return user, nil
	}
	return nil, ErrNotFound
}
// ByUsername finds a user by their username.