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/pkg/consts/consts.go

@@ -185,6 +185,32 @@ func MediaAssetStatusItems() []requests.KV {
 	return items
 }
 
+// swagger:enum MediaAssetVariant
+// ENUM( main, preview )
+type MediaAssetVariant string
+
+// Description returns the Chinese label for the specific enum value.
+func (t MediaAssetVariant) Description() string {
+	switch t {
+	case MediaAssetVariantMain:
+		return "正片产物"
+	case MediaAssetVariantPreview:
+		return "试看产物"
+	default:
+		return "未知产物"
+	}
+}
+
+// MediaAssetVariantItems returns the KV list for FE dropdowns.
+func MediaAssetVariantItems() []requests.KV {
+	values := MediaAssetVariantValues()
+	items := make([]requests.KV, 0, len(values))
+	for _, v := range values {
+		items = append(items, requests.NewKV(string(v), v.Description()))
+	}
+	return items
+}
+
 // contents
 
 // swagger:enum ContentStatus