fix: align private and channel update semantics

(cherry picked from commit c65f76f56278f74082c4fa792ed49104d5d33c38)

internal/domain/channel.go

@@ -347,6 +347,8 @@ type ChannelMessage struct {
 	Reactions    *ChannelMessageReactions
 	Action       *ChannelMessageAction
 	Media        *MessageMedia
+	Mentioned    bool
+	MediaUnread  bool
 	Pts          int
 	Deleted      bool
 }
@@ -827,6 +829,12 @@ type ChannelDifference struct {
 	Timeout      int
 }
 
+// DirtyChannel identifies an active channel with channel-scoped updates after an account difference date.
+type DirtyChannel struct {
+	ChannelID int64
+	Pts       int
+}
+
 // CreateChannelRequest creates a broadcast channel or megagroup.
 type CreateChannelRequest struct {
 	CreatorUserID int64