A list is a standing set of short strings under a tag. Not a task, because
milk is not work and the prioritiser must not count it as an errand; not a
fact, because it claims nothing. Nothing predicates over it, so two people
adding to the same list at once costs nothing.
Migration #19, plus AddListItem, ListItems, SetListItemStatus and ClearList.
The live-only unique index is the tasks one, per list: молоко twice before
the shop is one row, молоко again after it was crossed off is a new one.
