Simplify tagging

[snejus]

Refactor item tagging and fix several underlying issues.

Fixes

• Synchronise all artist list fields. Notably, artist_sort / artists_sort and artist_credit / artists_credit fields have not been synchronised.
• Fix overwrite_null configuration which was previously ignored for fields defined in autotag/__init__.py::SPECIAL_FIELDS.

Updates

• Move metadata application logic into Match objects: add Match.apply_metadata, AlbumMatch.apply_metadata, AlbumMatch.apply_album_metadata, and TrackMatch.apply_metadata; callers now use those methods instead of legacy free functions.
• Remove legacy functions from beets.autotag.__init__ (apply_item_metadata, apply_album_metadata, apply_metadata) and related globals (SPECIAL_FIELDS, log), and export only core types (AlbumInfo, AlbumMatch, TrackInfo, TrackMatch, Proposal, Recommendation, tag_album, tag_item).
• Add structured metadata facilities to Info and subclasses:
  • Info.type class property and nullable_fields for per-type 'overwrite_null' config.
  • Info.raw_data and Info.item_data computed properties to apply artist_credit rules, filter nulls, and map media-specific field names.
  • AlbumInfo and TrackInfo extend raw_data/item_data behavior to handle album/track specifics (date zeroing, tracktotal, mb_releasetrackid, per-disc numbering).
• Introduce TrackInfo.merge_with_album to merge track-level data with album-level fallback for a final item payload.
• Move correct_list_fields to hooks.py and update it to keep unmapped / non-media single/list fields in sync (artist <-> artists, albumtype <-> albumtypes, etc.).
• Wire changes through the codebase:
  • Pass Item objects into TrackMatch in match.tag_item to enable item-level metadata application.
  • Replace calls to removed autotag apply functions with Match.apply_metadata invocations in beets/importer/tasks.py, beetsplug/bpsync.py, and beetsplug/mbsync.py.
  • Update importer logic to set album artist fallbacks for albumartists / mb_albumartistids when missing.
• Add and update tests:
  • New test/autotag/test_hooks.py and test/autotag/test_match.py to validate new data mapping, list field synchronization, overwrite behavior, and assignment logic.

[github-actions[bot]]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.

[codecov[bot]]

Codecov Report

:x: Patch coverage is 95.45455% with 5 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 68.26%. Comparing base (60b4a38) to head (cf31d00). :warning: Report is 1 commits behind head on master. :white_check_mark: All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
beets/autotag/hooks.py	97.70%	1 Missing and 1 partial :warning:
beetsplug/bpsync.py	50.00%	2 Missing :warning:
beets/importer/tasks.py	92.30%	0 Missing and 1 partial :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6165   +/-   ##
=======================================
  Coverage   68.26%   68.26%           
=======================================
  Files         138      138           
  Lines       18791    18766   -25     
  Branches     3167     3158    -9     
=======================================
- Hits        12827    12811   -16     
+ Misses       5290     5286    -4     
+ Partials      674      669    -5     

Files with missing lines	Coverage Δ
beets/autotag/__init__.py	72.72% <ø> (-14.89%)	:arrow_down:
beets/autotag/match.py	76.92% <100.00%> (ø)
beetsplug/mbsync.py	82.05% <100.00%> (+0.23%)	:arrow_up:
beets/importer/tasks.py	90.83% <92.30%> (-0.06%)	:arrow_down:
beets/autotag/hooks.py	98.97% <97.70%> (-1.03%)	:arrow_down:
beetsplug/bpsync.py	19.04% <50.00%> (+0.97%)	:arrow_up:

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[snejus]

@semohr @henry-oberholtzer @JOJ0 would be happy to have your eyes over this as I think this is a significant piece of work making autotagging somewhat simpler (hopefully!).

[JOJ0]

@semohr @henry-oberholtzer @JOJ0 would be happy to have your eyes over this as I think this is a significant piece of work making autotagging somewhat simpler (hopefully!).

I'll try to find some time after first wave of xmadness is over