Replaces the blind fuzzy search with a smart hierarchy that uses the data we
already have, best identity first:
1. ISRC embedded in the file tags (enriched track) -> exact track.
2. Album -> track: use the album's stored source ID (albums.spotify_album_id /
itunes_album_id / deezer_id / musicbrainz_release_id / audiodb_id) when the
ALBUM is enriched (even if the track isn't); else find the album by searching
'artist album', then locate our track in that album's tracklist by normalized
title (track_number breaks ties). Pins the exact album context. (artist->album->track)
3. Plain artist+title search with similarity scoring. (artist->track) — loosest.
_load_tracks now returns dict rows (adds track_number + the album source-id
columns). Findings record matched_via = isrc | album | search. All clients
(spotify/deezer/itunes/discogs) expose search_albums + get_album_tracks with a
uniform {'items': [...]} shape, so the album tier is source-agnostic.
26 repair tests pass (added album-tier + _find_track_in_album coverage).
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