usernames: report reserved names as taken in the check paths too

account.checkUsername / channels.checkUsername / bots.checkUsername said a
reserved name was available and only updateUsername rejected it. Add the
blocklist check to peerUsernameAvailable (covers account + channel, both
backends) and to bots.Service.CheckUsername, so the client shows "username is
taken" immediately.

internal/store/memory/users.go

@@ -158,6 +158,9 @@ func (s *UserStore) CheckUsername(_ context.Context, userID int64, username stri
 	if username == "" {
 		return true, nil
 	}
+	if s.usernameRegistry != nil && s.usernameRegistry.nameReserved(username) {
+		return false, nil
+	}
 	s.mu.RLock()
 	defer s.mu.RUnlock()
 	for id, u := range s.byID {