Create a media asset

Downloads the media at the given URL and stores it in APIFRAME storage. Creation is synchronous — the asset is immediately Active and can be referenced as asset://{assetId} in any media URL field of a generation request (images, videos, music, edit and upscale endpoints); the API resolves it to the stored URL before dispatching the job. Unlike /v2/uploads files, assets do not expire. Limits: 25 MB for images/audio, 50 MB for video; the declared type must match the actual file contents (magic-byte sniffed).