Navidrome and gonic expose OpenSubsonic discTitles on the album response.
Parse them into a transient Album.discTitles map and persist each album's
titles in a child discs table (sourceId, albumId, disc, title) cascading
from albums. Songs keep only their disc number, and the album screen joins
by disc to render a title when one exists, falling back to "Disc N".
Titles are read opportunistically: a server that doesn't send discTitles
just leaves the table empty, so no API version negotiation is needed.
Titles apply to single- and multi-disc albums, are pruned when removed,
and are stored per album.
Schema 10 -> 11.
