fix: resolve err113 lint issues with sentinel errors

internal/models/channel_member.go

@@ -2,7 +2,6 @@ package models
 
 import (
 	"context"
-	"errors"
 	"time"
 )
 
@@ -23,7 +22,7 @@ func (cm *ChannelMember) User(ctx context.Context) (*User, error) {
 		return ul.GetUserByID(ctx, cm.UserID)
 	}
 
-	return nil, errors.New("user lookup not available")
+	return nil, ErrUserLookupNotAvailable
 }
 
 // Channel returns the full Channel for this membership.
@@ -32,5 +31,5 @@ func (cm *ChannelMember) Channel(ctx context.Context) (*Channel, error) {
 		return cl.GetChannelByID(ctx, cm.ChannelID)
 	}
 
-	return nil, errors.New("channel lookup not available")
+	return nil, ErrChannelLookupNotAvailable
 }