Create a private collection of base references.

Collections store references, not copied content. They start private. Owner search reads current files in owned bases and published files in other bases. Publication includes only references to currently public bases; private bases and later additions stay out. Every public read checks base publication again. Private or withdrawn collections return 404. Responses use private, no-store. Returns: { collection{id,name,visibility,created_at,api,public_url} }