A downloaded track was getting the wrong track number (e.g. 'Apologize' from Shock
Value tagged track 1 instead of 16). Root cause: Deezer PLAYLIST track objects don't
carry `track_position` (only `/track/<id>` and `/album/<id>/tracks` do — even the
album object's embedded tracks omit it, verified against the live API). Both Deezer
playlist builders numbered tracks by their enumerate INDEX, which poisoned the real
album track number — and that value rides into the wishlist and onto the file tag.
Resolve the authoritative position from each album's `/album/<id>/tracks` (cache-first,
handles multi-disc) via a shared resolve_album_track_positions() helper, used by both
deezer_client.get_playlist and deezer_download_client.get_playlist_tracks. Falls back
to the index only if the album lookup fails (no regression). Seam tests for the helper.
