perf: sync megagroup unread derivation

internal/store/postgres/channel_message_delete.go

@@ -411,9 +411,8 @@ RETURNING pinned_message_id`, channel.ID, topID, pts).Scan(&latestPinned); err !
 	channel.TopMessageID = topID
 	channel.Pts = pts
 	channel.PinnedMessageID = latestPinned
-	if err := refreshChannelDialogsAfterDeleteTx(ctx, tx, channel); err != nil {
-		return nil, domain.ChannelUpdateEvent{}, channel, err
-	}
+	// 删除后不再对全员刷新 channel_dialogs 缓存行（性能审计 H4a）：unread 读时由
+	// 可见 incoming 消息动态派生（deleted 自动出列），top 由 channels.top_message_id 提供。
 	event := domain.ChannelUpdateEvent{
 		ChannelID:    channel.ID,
 		Type:         domain.ChannelUpdateDeleteMessages,