fix: resolve nlreturn, modernize, perfsprint, wsl_v5, and partial err113 lint issues

internal/models/channel_member.go

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