fix(test): 修复CI测试失败

- 修复gofmt格式问题
- 为stubUsageLogRepo添加缺失的GetApiKeyStatsAggregated方法

backend/internal/server/api_contract_test.go

@@ -922,6 +922,10 @@ func (r *stubUsageLogRepo) GetUserUsageTrend(ctx context.Context, startTime, end
 	return nil, errors.New("not implemented")
 }
 
+func (r *stubUsageLogRepo) GetApiKeyStatsAggregated(ctx context.Context, apiKeyID int64, startTime, endTime time.Time) (*usagestats.UsageStats, error) {
+	return nil, errors.New("not implemented")
+}
+
 func (r *stubUsageLogRepo) GetBatchUserUsageStats(ctx context.Context, userIDs []int64) (map[int64]*usagestats.BatchUserUsageStats, error) {
 	return nil, errors.New("not implemented")
 }