Artist biographies come from getArtistInfo2, which the library sync does
not call. Store them in their own table so a page open can use the last
value immediately and downloads can keep it for offline use, without
pulling a large text column into every artist list query.
A missing row means the artist has never been fetched; a row with a null
biography means the server had none. That lets the page show the loading
state only until the first fetch completes.
