fix(lint): 修复 golangci-lint 检查问题

- 格式化代码 (gofmt)
- 修复 rows.Close() 返回值未检查 (errcheck)
- 删除未使用的 usage_clamp.go 文件 (unused)
- 删除临时测试目录

backend/internal/repository/account_repo.go

@@ -975,7 +975,7 @@ func (r *accountRepository) loadTempUnschedStates(ctx context.Context, accountID
 	if err != nil {
 		return nil, err
 	}
-	defer rows.Close()
+	defer func() { _ = rows.Close() }()
 
 	for rows.Next() {
 		var id int64

backend/internal/repository/user_repo.go

@@ -301,7 +301,7 @@ func (r *userRepository) filterUsersByAttributes(ctx context.Context, attrs map[
 	if err != nil {
 		return nil, err
 	}
-	defer rows.Close()
+	defer func() { _ = rows.Close() }()
 
 	result := make([]int64, 0)
 	for rows.Next() {