fix: resolve err113 lint issues with sentinel errors

internal/models/session.go

@@ -2,7 +2,6 @@ package models
 
 import (
 	"context"
-	"errors"
 	"time"
 )
 
@@ -23,5 +22,5 @@ func (s *Session) User(ctx context.Context) (*User, error) {
 		return ul.GetUserByID(ctx, s.UserID)
 	}
 
-	return nil, errors.New("user lookup not available")
+	return nil, ErrUserLookupNotAvailable
 }