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/channel_settings.go

@@ -128,6 +128,9 @@ func (s *ChannelStore) CheckUsername(_ context.Context, userID, channelID int64,
 		return false, err
 	}
 	usernameLower := strings.ToLower(strings.TrimSpace(strings.TrimPrefix(username, "@")))
+	if s.usernameRegistry != nil && s.usernameRegistry.nameReserved(usernameLower) {
+		return false, nil
+	}
 	for id, channel := range s.channels {
 		if channel.Deleted || channel.Username == "" {
 			continue