fix(test): add missing stub methods for RedeemCodeRepository and AdminService

Add ListByUserPaginated and SumPositiveBalanceByUser to redeemRepoStub
Add GetUserBalanceHistory to stubAdminService

Fixes CI test compilation errors

backend/internal/handler/admin/admin_service_stub_test.go

@@ -290,5 +290,9 @@ func (s *stubAdminService) ExpireRedeemCode(ctx context.Context, id int64) (*ser
 	return &code, nil
 }
 
+func (s *stubAdminService) GetUserBalanceHistory(ctx context.Context, userID int64, page, pageSize int, codeType string) ([]service.RedeemCode, int64, float64, error) {
+	return s.redeems, int64(len(s.redeems)), 100.0, nil
+}
+
 // Ensure stub implements interface.
 var _ service.AdminService = (*stubAdminService)(nil)