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/admin/usage_handler.go

@@ -4,6 +4,7 @@ import (
 	"strconv"
 	"time"
 
+	"github.com/Wei-Shaw/sub2api/internal/handler/dto"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/pagination"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/response"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/timezone"
@@ -94,7 +95,11 @@ func (h *UsageHandler) List(c *gin.Context) {
 		return
 	}
 
-	response.Paginated(c, records, result.Total, page, pageSize)
+	out := make([]dto.UsageLog, 0, len(records))
+	for i := range records {
+		out = append(out, *dto.UsageLogFromService(&records[i]))
+	}
+	response.Paginated(c, out, result.Total, page, pageSize)
 }
 
 // Stats handles getting usage statistics with filters