Update or create a branding asset blob. Requires admin authorization.

Branding asset key (mainLogo, favicon, siteTitle, etc.)

DID of the broadcaster. If not provided, uses the server's default broadcaster.

Base64-encoded blob data

MIME type of the blob (e.g., image/png, text/plain)

Image width in pixels (optional, for images only)

Image height in pixels (optional, for images only)