Follow-up to the prior compilation-album scanner fix. That patch
made the scanner read `tracks.track_artist` (per-track artist
column) via COALESCE so compilation tracks would compare against
the right value. But tracks downloaded BEFORE the `track_artist`
column existed have track_artist=NULL — COALESCE falls back to
album artist (the curator) and the wrong-comparison case returns.
Fix: explicit 3-tier resolution in `_scan_file`:
1. DB `tracks.track_artist` if populated → trust it. Respects
manual edits from the enhanced library view (user who curated
the DB value but didn't re-tag the file gets their edit
respected, not overridden by stale file tag).
2. File's ARTIST tag via mutagen if present → use it. Tidal /
Spotify / Deezer all write the per-track artist into the
audio file at download time regardless of SoulSync's DB
schema, so it's ground truth even when the DB column is
stale or NULL. File is already open for fingerprinting so
mutagen tag-read is essentially free.
3. Album artist → final fallback for files without proper ARTIST
tags AND no DB track_artist. Existing pre-fix behavior.
`_load_db_tracks` SELECT now surfaces `track_artist` (raw, may be
empty/NULL via NULLIF) and `album_artist` separately in addition
to the COALESCE'd `artist` field — so `_scan_file` can tell the
difference between 'DB has a curated value' and 'DB fell back to
album artist'. Without this distinction, the file-tag fallback
would create false positives when DB is curated but file is stale.
5 new tests (11 total in the file) pin:
- File-tag-trumps-DB resolves the legacy NULL case (DB says
'Andromedik' (album curator), file says 'Eclypse', AcoustID
says 'Eclypse' → no flag)
- Tag-missing falls back to album artist (preserves existing
genuine-mismatch contract — file without tag + AcoustID
mismatch still flags)
- Mutagen exception swallowed (debug log, fall-through)
- File-tag matches DB → no behavioral change
- DB curated value trumps stale file tag (false-positive guard
— user edited DB without re-tagging file shouldn't get flagged)
Two existing test fixtures (`_make_context` callers) updated to
the new 10-column row shape.
SQL behavior verified empirically against real SQLite: NULL and
empty-string both flow through NULLIF → None in Python →
file-tag-fallback path. Modern populated values trump file tag.
7a23d60f28