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/app/services/media_asset_test.go

@@ -53,9 +53,10 @@ func (s *MediaAssetTestSuite) Test_AdminUploadInit_VariantAndSource() {
 
 		Convey("main variant 不允许 source_asset_id", func() {
 			src := int64(123)
+			v := consts.MediaAssetVariantMain
 			_, err := MediaAsset.AdminUploadInit(ctx, tenantID, userID, &tenant_dto.AdminMediaAssetUploadInitForm{
 				Type:          "video",
-				Variant:       "main",
+				Variant:       &v,
 				SourceAssetID: &src,
 			}, now)
 			So(err, ShouldNotBeNil)
@@ -65,9 +66,10 @@ func (s *MediaAssetTestSuite) Test_AdminUploadInit_VariantAndSource() {
 		})
 
 		Convey("preview variant 必须带 source_asset_id", func() {
+			v := consts.MediaAssetVariantPreview
 			_, err := MediaAsset.AdminUploadInit(ctx, tenantID, userID, &tenant_dto.AdminMediaAssetUploadInitForm{
 				Type:    "video",
-				Variant: "preview",
+				Variant: &v,
 			}, now)
 			So(err, ShouldNotBeNil)
 			var appErr *errorx.AppError
@@ -94,9 +96,10 @@ func (s *MediaAssetTestSuite) Test_AdminUploadInit_VariantAndSource() {
 			_, err := s.DB.ExecContext(ctx, "UPDATE media_assets SET variant = 'preview' WHERE tenant_id = $1 AND id = $2", tenantID, src.ID)
 			So(err, ShouldBeNil)
 
+			v := consts.MediaAssetVariantPreview
 			_, err = MediaAsset.AdminUploadInit(ctx, tenantID, userID, &tenant_dto.AdminMediaAssetUploadInitForm{
 				Type:          "video",
-				Variant:       "preview",
+				Variant:       &v,
 				SourceAssetID: &src.ID,
 			}, now)
 			So(err, ShouldNotBeNil)
@@ -106,4 +109,3 @@ func (s *MediaAssetTestSuite) Test_AdminUploadInit_VariantAndSource() {
 		})
 	})
 }
-