feat: 新增批量获取文件下载预签名 URL 接口

新增 POST /api/admin/storage/batch-download-urls，支持一次传入最多 50 个 file_key，返回对应预签名下载 URL 映射。解决列表页批量展示图片需多次请求的问题。

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

internal/handler/admin/storage.go

@@ -19,6 +19,42 @@ func NewStorageHandler(service *storage.Service) *StorageHandler {
 	return &StorageHandler{service: service}
 }
 
+// GetBatchDownloadURLs 批量获取文件下载预签名 URL
+// POST /api/admin/storage/batch-download-urls
+func (h *StorageHandler) GetBatchDownloadURLs(c *fiber.Ctx) error {
+	if h.service == nil {
+		return errors.New(errors.CodeInternalError, "对象存储服务未配置")
+	}
+
+	var req dto.GetBatchDownloadURLsRequest
+	if err := c.BodyParser(&req); err != nil {
+		return errors.New(errors.CodeInvalidParam)
+	}
+
+	urls := make(map[string]string, len(req.FileKeys))
+	expiresIn := 0
+
+	for _, fileKey := range req.FileKeys {
+		result, err := h.service.GetDownloadURL(c.UserContext(), fileKey)
+		if err != nil {
+			logger.GetAppLogger().Error("获取下载URL失败",
+				zap.String("file_key", fileKey),
+				zap.Error(err),
+			)
+			return errors.New(errors.CodeInternalError, "获取下载URL失败")
+		}
+		urls[fileKey] = result.URL
+		expiresIn = result.ExpiresIn
+	}
+
+	return response.Success(c, dto.GetBatchDownloadURLsResponse{
+		URLs:      urls,
+		ExpiresIn: expiresIn,
+	})
+}
+
+// GetUploadURL 获取文件上传预签名 URL
+// POST /api/admin/storage/upload-url
 func (h *StorageHandler) GetUploadURL(c *fiber.Ctx) error {
 	if h.service == nil {
 		return errors.New(errors.CodeInternalError, "对象存储服务未配置")