show album disc titles instead of generic disc numbers

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.

app/src/main/kotlin/com/subtracks/ui/library/AlbumDetailViewModel.kt

@@ -4,6 +4,7 @@ import androidx.lifecycle.ViewModel
 import androidx.lifecycle.viewModelScope
 import com.subtracks.data.model.Album
 import com.subtracks.data.model.CoverArtRef
+import com.subtracks.data.model.Disc
 import com.subtracks.data.model.Song
 import com.subtracks.data.repo.LibraryRepository
 import com.subtracks.data.repo.SourceRepository
@@ -28,6 +29,8 @@ class AlbumDetailViewModel(
 
     val songs: Flow<List<Song>> = sourceId.flatMapLatest { libraryRepository.albumSongs(it, albumId) }
 
+    val discs: Flow<List<Disc>> = sourceId.flatMapLatest { libraryRepository.albumDiscs(it, albumId) }
+
     fun coverArt(
         coverArt: String?,
         thumbnail: Boolean,