feat: 添加媒体资产变体和来源ID字段，支持预览产品与主资产的关联

backend/app/http/tenant/dto/media_asset_admin.go

@@ -7,6 +7,14 @@ type AdminMediaAssetUploadInitForm struct {
 	// Type is the media asset type (video/audio/image).
 	// Used to decide processing pipeline and validation rules; required.
 	Type string `json:"type,omitempty"`
+
+	// Variant indicates whether this asset is a main or preview product.
+	// Allowed: main/preview; default is main.
+	Variant string `json:"variant,omitempty"`
+
+	// SourceAssetID links a preview product to its main asset; only meaningful when variant=preview.
+	SourceAssetID *int64 `json:"source_asset_id,omitempty"`
+
 	// ContentType is the MIME type reported by the client (e.g. video/mp4); optional.
 	// Server should not fully trust it, but can use it as a hint for validation/logging.
 	ContentType string `json:"content_type,omitempty"`