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/database/.transform.yaml

@@ -22,6 +22,7 @@ field_type:
   media_assets:
     type: consts.MediaAssetType
     status: consts.MediaAssetStatus
+    variant: consts.MediaAssetVariant
   contents:
     status: consts.ContentStatus
     visibility: consts.ContentVisibility