channels: clear stale banned rights when promoting a member to admin

EditChannelAdmin reactivates a previously kicked/banned member to Active
status but left BannedRights.ViewMessages set from the old ban, an
Active+banned-from-viewing state nothing else in the codebase expects.
TransferChannelOwnership's participant check trips on it and rejects the
transfer with PARTICIPANT_MISSING, even though the target is now a
visibly active admin.

internal/store/postgres/channel_member_admin.go

@@ -100,6 +100,13 @@ func (s *ChannelStore) EditChannelAdmin(ctx context.Context, req domain.EditChan
 	member.InviterUserID = req.UserID
 	member.Status = domain.ChannelMemberActive
 	member.LeftAt = 0
+	// A kicked/banned member carries BannedRights.ViewMessages=true (see
+	// EditChannelBanned above); editAdmin unconditionally reactivates them
+	// to Active, so any lingering ban must be lifted too -- otherwise the
+	// contradictory Active+ViewMessages-banned state trips later checks
+	// (e.g. TransferChannelOwnership's participant check) that never
+	// expect an active member to be ban-restricted from viewing messages.
+	member.BannedRights = domain.ChannelBannedRights{}
 	if req.HasRank() {
 		member.Rank = req.Rank
 	}