Image cleanup looked each file up in a list of image ids, an import added
up its results by copying every issue so far on each entry, and every item
on a page worked out its Copy to next/previous page menu by sorting the
pages and scanning both neighbours as it built. Each grew with the square
of the data.

Cleanup uses a set; imports keep one running total; the copy menus read
the neighbouring pages' ids from an index built once per saved strategy
and page on screen (local) or once per version of the page's items
(cloud).