fix: admin handlers 添加 DTO 转换修复 JSON 序列化

修复 PR #36 合并后部分 admin handler 直接返回 service 层对象导致
JSON 字段名为 PascalCase 而非期望的 snake_case 问题。

修复内容:
- account_handler: Refresh 接口添加 dto.AccountFromService
- openai_oauth_handler: RefreshAccountToken/CreateAccountFromOAuth 添加 dto 转换
- subscription_handler: BulkAssign 添加 dto.BulkAssignResultFromService
- usage_handler: List 接口添加 dto.UsageLogFromService 转换
- 新增 dto.BulkAssignResult 类型和对应的 mapper 函数

backend/internal/handler/dto/mappers.go

@@ -292,3 +292,19 @@ func UserSubscriptionFromService(sub *service.UserSubscription) *UserSubscriptio
 		AssignedByUser:     UserFromServiceShallow(sub.AssignedByUser),
 	}
 }
+
+func BulkAssignResultFromService(r *service.BulkAssignResult) *BulkAssignResult {
+	if r == nil {
+		return nil
+	}
+	subs := make([]UserSubscription, 0, len(r.Subscriptions))
+	for i := range r.Subscriptions {
+		subs = append(subs, *UserSubscriptionFromService(&r.Subscriptions[i]))
+	}
+	return &BulkAssignResult{
+		SuccessCount:  r.SuccessCount,
+		FailedCount:   r.FailedCount,
+		Subscriptions: subs,
+		Errors:        r.Errors,
+	}
+}

backend/internal/handler/dto/types.go

@@ -210,3 +210,10 @@ type UserSubscription struct {
 	Group          *Group `json:"group,omitempty"`
 	AssignedByUser *User  `json:"assigned_by_user,omitempty"`
 }
+
+type BulkAssignResult struct {
+	SuccessCount  int                `json:"success_count"`
+	FailedCount   int                `json:"failed_count"`
+	Subscriptions []UserSubscription `json:"subscriptions"`
+	Errors        []string           `json:"errors"`
+}