app.zeens.getSubjectCovers

Cover thumbnails for a batch of zeen / collection URIs. Deliberately narrow -- URIs in, cover refs out -- so a notifications list can draw a preview per row without a round trip each. getCollection takes one uri and returns the whole record plus an item count; a zeen had no AppView read at all and getZeen goes to the author's PDS for every page's full text to find one cid.