Create a render directly from provided assets (no preprocessing/training).

Each side is either a pre-ingested id or a source URL (exactly one per side). When both sides are ids
the render is created + dispatched synchronously (201 {generate_id, credit}). When either side
is a URL the assets are ingested asynchronously and the render fires once both downloads complete
(202 {tracking_id}); poll GET /v1/renders/track/{tracking_id}.

Args:
    add_render (CreateDirectRenderModel): video_id|video_url, audio_id|audio_url, output_filename,
        callback_url

Returns:
    Dict[str, Any]: sync → ``{data: {generate_id, credit}}``; async → ``{data: {tracking_id}}``