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.json

@@ -2787,6 +2787,17 @@
                 "MediaAssetTypeImage"
             ]
         },
+        "consts.MediaAssetVariant": {
+            "type": "string",
+            "enum": [
+                "main",
+                "preview"
+            ],
+            "x-enum-varnames": [
+                "MediaAssetVariantMain",
+                "MediaAssetVariantPreview"
+            ]
+        },
         "consts.OrderStatus": {
             "type": "string",
             "enum": [
@@ -3050,7 +3061,11 @@
                 },
                 "variant": {
                     "description": "Variant indicates whether this asset is a main or preview product.\nAllowed: main/preview; default is main.",
-                    "type": "string"
+                    "allOf": [
+                        {
+                            "$ref": "#/definitions/consts.MediaAssetVariant"
+                        }
+                    ]
                 }
             }
         },
@@ -4055,6 +4070,10 @@
                     "description": "存储提供方：例如 s3/minio/oss；便于多存储扩展",
                     "type": "string"
                 },
+                "source_asset_id": {
+                    "description": "派生来源资源ID：preview 产物可指向对应 main 资源；用于建立 preview/main 的 1:1 追溯关系",
+                    "type": "integer"
+                },
                 "status": {
                     "description": "处理状态：uploaded/processing/ready/failed/deleted；ready 才可被内容引用对外提供",
                     "allOf": [
@@ -4082,6 +4101,14 @@
                 "user_id": {
                     "description": "用户ID：资源上传者；用于审计与权限控制",
                     "type": "integer"
+                },
+                "variant": {
+                    "description": "产物类型：main/preview；用于强制试看资源必须绑定独立产物，避免用正片绕过",
+                    "allOf": [
+                        {
+                            "$ref": "#/definitions/consts.MediaAssetVariant"
+                        }
+                    ]
                 }
             }
         },