New endpoint — Delete a protected asset

Added DELETE /protected-asset/{id} to delete a protected asset by ID. The asset is removed from your organization's active protected assets and related designated takedown assets are disabled — matching the existing UI behavior.

Returns 200 with the deleted asset ID on success, 400 if the ID is not a valid UUID, or 404 if the asset is not found, does not belong to your organization, or has already been deleted.