Simplify multi-genre implementation based on maintainer feedback (PR #6169).
Changes:
- Remove multi_value_genres and genre_separator config options
- Replace complex sync_genre_fields() with ensure_first_value('genre', 'genres')
- Update all plugins (Beatport, MusicBrainz, LastGenre) to always write genres as lists
- Add automatic migration for comma/semicolon/slash-separated genre strings
- Add 'beet migrate genres' command for explicit batch migration with --pretend flag
- Update all tests to reflect simplified approach (44 tests passing)
- Update documentation
Implementation aligns with maintainer vision of always using multi-value genres
internally with automatic backward-compatible sync to the genre field via
ensure_first_value(), eliminating configuration complexity.
Migration strategy avoids problems from #5540:
- Automatic lazy migration on item access (no reimport/mbsync needed)
- Optional batch migration command for user control
- No endless rewrite loops due to proper field synchronization
Introduce a new RequestHandler base class to introduce a shared session,
centralize HTTP request management and error handling across plugins.
Key changes:
- Add RequestHandler base class with a shared/cached session
- Convert TimeoutSession to use SingletonMeta for proper resource
management
- Create LyricsRequestHandler subclass with lyrics-specific error
handling
- Update MusicBrainzAPI to inherit from RequestHandler
- Instead of checking for empty `artist` query, use `va_likely`
parameter to determine whether we should query for Various Artists or
not.
- `album` / `title` is always a truthy string - no need to handle empty
criteria case
- `tracks` list always has at least one track - no need to check for
`len(items)`
