feat: Introduce MediaAssetVariant for better asset management

- Added MediaAssetVariant enum with values 'main' and 'preview'.
- Updated media asset service logic to utilize MediaAssetVariant for variant handling.
- Refactored database models and queries to include variant and source_asset_id fields.
- Enhanced validation for asset variants in upload and processing functions.
- Updated Swagger documentation to reflect new variant structure and descriptions.
- Implemented necessary database migrations to support the new variant constraints.

backend/docs/swagger.yaml

@@ -84,6 +84,14 @@ definitions:
     - MediaAssetTypeVideo
     - MediaAssetTypeAudio
     - MediaAssetTypeImage
+  consts.MediaAssetVariant:
+    enum:
+    - main
+    - preview
+    type: string
+    x-enum-varnames:
+    - MediaAssetVariantMain
+    - MediaAssetVariantPreview
   consts.OrderStatus:
     enum:
     - created
@@ -299,10 +307,11 @@ definitions:
           Used to decide processing pipeline and validation rules; required.
         type: string
       variant:
+        allOf:
+        - $ref: '#/definitions/consts.MediaAssetVariant'
         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:
@@ -976,6 +985,9 @@ definitions:
       provider:
         description: 存储提供方：例如 s3/minio/oss；便于多存储扩展
         type: string
+      source_asset_id:
+        description: 派生来源资源ID：preview 产物可指向对应 main 资源；用于建立 preview/main 的 1:1 追溯关系
+        type: integer
       status:
         allOf:
         - $ref: '#/definitions/consts.MediaAssetStatus'
@@ -993,6 +1005,10 @@ definitions:
       user_id:
         description: 用户ID：资源上传者；用于审计与权限控制
         type: integer
+      variant:
+        allOf:
+        - $ref: '#/definitions/consts.MediaAssetVariant'
+        description: 产物类型：main/preview；用于强制试看资源必须绑定独立产物，避免用正片绕过
     type: object
   models.Order:
     properties: