address review: NULL disc/track in backward album seeks; drop redundant index

- `albumSongsBefore` used the null-intolerant row-value `<`, so an album
  with NULL `disc`/`track` (untagged files) dropped exactly the rows a
  backward window needs, returning the wrong track at a position. Use a
  NULL-aware predicate (NULL sorts first, so it belongs to any `before`
  set); `albumSongsFrom` is unaffected.
- The new `(sourceId, albumId, disc, track, id)` index has the old
  `(sourceId, albumId)` index as a prefix, so drop the latter in the
  migration and from the entity.
- Tests: mixed NULL disc/track album for the before-window and a full
  backward scan; both fail on the previous commit.

app/src/main/kotlin/com/subtracks/data/db/Migrations.kt

@@ -266,6 +266,8 @@ val MIGRATION_18_19 =
                 "CREATE INDEX IF NOT EXISTS `index_songs_sourceId_albumId_order` ON `songs` " +
                     "(`sourceId`, `albumId`, `disc`, `track`, `id`)",
             )
+            // Covered by the prefix of the new index.
+            connection.execSQL("DROP INDEX IF EXISTS `index_songs_sourceId_albumId`")
         }
     }
 

app/src/main/kotlin/com/subtracks/data/db/QueueDao.kt

@@ -125,8 +125,13 @@ interface QueueDao {
         limit: Int,
     ): List<SongListItem>
 
+    // Row-value `<` is null-intolerant, and NULL disc/track sort first, so a plain
+    // `(disc, track, id) < (...)` would drop exactly the untagged tracks a backward window needs.
     @Query(
-        "$ALBUM_SONGS_SELECT AND (songs.disc, songs.track, songs.id) < (:disc, :track, :id) " +
+        "$ALBUM_SONGS_SELECT AND (" +
+            "songs.disc IS NULL OR songs.disc < :disc OR " +
+            "(songs.disc = :disc AND (songs.track IS NULL OR songs.track < :track OR " +
+            "(songs.track = :track AND songs.id < :id)))) " +
             "ORDER BY songs.disc DESC, songs.track DESC, songs.id DESC LIMIT :limit OFFSET :skip",
     )
     suspend fun albumSongsBefore(

app/src/main/kotlin/com/subtracks/data/model/Models.kt

@@ -210,7 +210,6 @@ data class PlaylistSong(
         ),
     ],
     indices = [
-        Index("sourceId", "albumId"),
         Index("sourceId", "artistId"),
         Index(
             name = "index_songs_sourceId_albumId_order",