fix(test): 添加测试辅助函数 truncateToDayUTC 修复编译错误

在 usage_log_repo_integration_test.go 中添加本地的 truncateToDayUTC
辅助函数，修复因主代码重命名该函数导致的测试编译错误。
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/internal/repository/usage_log_repo_integration_test.go

@@ -36,6 +36,12 @@ func TestUsageLogRepoSuite(t *testing.T) {
 	suite.Run(t, new(UsageLogRepoSuite))
 }
 
+// truncateToDayUTC 截断到 UTC 日期边界（测试辅助函数）
+func truncateToDayUTC(t time.Time) time.Time {
+	t = t.UTC()
+	return time.Date(t.Year(), t.Month(), t.Day(), 0, 0, 0, 0, time.UTC)
+}
+
 func (s *UsageLogRepoSuite) createUsageLog(user *service.User, apiKey *service.APIKey, account *service.Account, inputTokens, outputTokens int, cost float64, createdAt time.Time) *service.UsageLog {
 	log := &service.UsageLog{
 		UserID:       user.ID,