feat: 添加媒体资产变体和来源ID字段，支持预览产品与主资产的关联

backend/docs/swagger.yaml

@@ -289,11 +289,20 @@ definitions:
           SHA256 is the hex-encoded sha256 of the file; optional.
           Used for deduplication/audit; server may validate it later during upload-complete.
         type: string
+      source_asset_id:
+        description: SourceAssetID links a preview product to its main asset; only
+          meaningful when variant=preview.
+        type: integer
       type:
         description: |-
           Type is the media asset type (video/audio/image).
           Used to decide processing pipeline and validation rules; required.
         type: string
+      variant:
+        description: |-
+          Variant indicates whether this asset is a main or preview product.
+          Allowed: main/preview; default is main.
+        type: string
     type: object
   dto.AdminMediaAssetUploadInitResponse:
     properties: