1. Fix filename parser pattern order — "01 - Title" now matched
before "Artist - Title", preventing track numbers being treated
as artist names (e.g., "08" no longer becomes the artist)
2. Tag priority over filename parsing — shared _read_staging_file_metadata()
helper reads title, artist, albumartist, album, track_number, disc_number
from Mutagen tags. Only falls back to filename parsing when BOTH title
AND artist tags are empty. Applied to all 3 staging scan sites.
3. Improved match scoring — rebalanced from title(0.5)+tracknum(0.5) to
title(0.45)+artist(0.15)+tracknum(0.30)+album_bonus(0.10). Files
whose album tag matches the selected album get boosted.
4. Auto-group detection — new /api/import/staging/groups endpoint groups
staging files by album+artist tags. Frontend shows "Auto-Detected
Albums" section with one-click search. Match endpoint accepts
optional file_paths filter to scope matching to a specific group.
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