@RequestBody String username) {
String cleaned = username.trim().toLowerCase();
User user = new User(cleaned, LocalDateTime.now(), false);
repository.save(user);
// TODO: Not-Unique Fehler auslösen
@RequestBody String username) {
String cleaned = username.trim().toLowerCase();
User user = new User(cleaned, LocalDateTime.now(), false);
repository.save(user);
// TODO: Not-Unique Fehler auslösen