New core/genre_filter.py with ~180 curated default genres. When strict
mode is enabled in Settings → Library Preferences → Genre Whitelist,
only whitelisted genres pass through during enrichment. Junk tags from
Last.fm (artist names, radio shows, playlist names) are silently dropped.
Applied at all 10 genre write points: Spotify, Last.fm, AudioDB, Deezer,
Discogs, iTunes, Qobuz enrichment workers + post-processing genre merge
+ initial download artist/album creation.
Strict mode is OFF by default — zero behavior change for existing users.
First enable auto-populates the whitelist with defaults. Users can add,
remove, search, and reset genres via the Settings UI.
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