List the question-template categories as a tree

Every category of the admin question-template library, nested: the root categories, each with its sub-categories under children (set only when it has some), ordered by idx at every level. A category whose parent no longer exists is listed at the root (legacy answered 500). Unpaginated, in the { data, metadata } envelope. Legacy answered { categories, questions } with every question template and its options; the admin pages read only the categories, so the questions are not carried over.