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/docs.go

@@ -2793,6 +2793,17 @@ const docTemplate = `{
                 "MediaAssetTypeImage"
             ]
         },
+        "consts.MediaAssetVariant": {
+            "type": "string",
+            "enum": [
+                "main",
+                "preview"
+            ],
+            "x-enum-varnames": [
+                "MediaAssetVariantMain",
+                "MediaAssetVariantPreview"
+            ]
+        },
         "consts.OrderStatus": {
             "type": "string",
             "enum": [
@@ -3056,7 +3067,11 @@ const docTemplate = `{
                 },
                 "variant": {
                     "description": "Variant indicates whether this asset is a main or preview product.\nAllowed: main/preview; default is main.",
-                    "type": "string"
+                    "allOf": [
+                        {
+                            "$ref": "#/definitions/consts.MediaAssetVariant"
+                        }
+                    ]
                 }
             }
         },
@@ -4061,6 +4076,10 @@ const docTemplate = `{
                     "description": "存储提供方：例如 s3/minio/oss；便于多存储扩展",
                     "type": "string"
                 },
+                "source_asset_id": {
+                    "description": "派生来源资源ID：preview 产物可指向对应 main 资源；用于建立 preview/main 的 1:1 追溯关系",
+                    "type": "integer"
+                },
                 "status": {
                     "description": "处理状态：uploaded/processing/ready/failed/deleted；ready 才可被内容引用对外提供",
                     "allOf": [
@@ -4088,6 +4107,14 @@ const docTemplate = `{
                 "user_id": {
                     "description": "用户ID：资源上传者；用于审计与权限控制",
                     "type": "integer"
+                },
+                "variant": {
+                    "description": "产物类型：main/preview；用于强制试看资源必须绑定独立产物，避免用正片绕过",
+                    "allOf": [
+                        {
+                            "$ref": "#/definitions/consts.MediaAssetVariant"
+                        }
+                    ]
                 }
             }
         },

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"
+                        }
+                    ]
                 }
             }
         },

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: