address review: offset fallback for far backward album seeks; pin the playlist keyset

- The NULL-aware backward album predicate is not sargable, so a backward seek
  from a far cursor scans the whole album partition. Fall back to the offset
  query when the anchor is more than twice the offset away; near-cursor
  backward reads still use keyset.
- Cover the playlist keyset path: a DAO test for `playlistSongsBefore` and a
  repository test scanning a playlist forward and backward. Both fail if the
  playlist order key stops driving the seek.

app/src/main/kotlin/com/subtracks/data/repo/QueueRepository.kt

@@ -764,6 +764,11 @@ class QueueRepository(
     ): List<SongItem>? {
         if (!cursor.key.seekable(entry.kind)) return null
         val from = cursor.ordinal
+        // A backward seek from a far cursor is not cheaper than the offset query: the album
+        // predicate is not sargable (it has to include NULL disc/track), so it scans the partition
+        // back to `offset` instead of using a range. Fall back when the anchor is more than twice
+        // the offset away, matching the documented far-jump behaviour.
+        if (from > offset && from - offset >= offset) return null
         return if (offset >= from) {
             rowsFrom(entry, cursor.key, offset - from, limit)
         } else {