fix(auth): allow admins to change any username including their own

services/api/internal/auth/authtest/memstore.go

@@ -293,7 +293,18 @@ func (s *MemStore) LockActiveAdmins(ctx context.Context) (int, error) { return s
 
 // UpdateUser реализует auth.Store.
 func (s *MemStore) UpdateUser(ctx context.Context, id uuid.UUID, upd auth.UserUpdate) (auth.User, error) {
+	if upd.Username != nil {
+		unlock := s.lock()
+		taken := s.usernameTaken(*upd.Username, id)
+		unlock()
+		if taken {
+			return auth.User{}, auth.ErrUsernameTaken
+		}
+	}
 	err := s.update(id, func(u *auth.User) {
+		if upd.Username != nil {
+			u.Username = *upd.Username
+		}
 		if upd.DisplayName != nil {
 			u.DisplayName = *upd.DisplayName
 		}