refactor(ftintitle): cache config options

[treyturner]

Pre-factor for #6726.

Description

Make ftintitle's config @cached_property attributes instead of passing them through the call stack. Modeled after similar recent changes in convert.

Changes

• Add cached plugin properties for auto, drop, format, keep_in_artist, preserve_album_artist, and custom_words, keeping the existing cached bracket_keywords property I forgot I added a while back.
• commands(), imported(), ft_in_title(), and update_metadata() are cleaned up to read these directly.
• Don't read auto during plugin init; the import stage remains registered and imported() checks auto when invoked.

To Do

• Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. (Very much encouraged but not strictly required.)

[github-actions]

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]

Codecov Report

❌ Patch coverage is 87.50000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.53%. Comparing base (ad03b35) to head (adc2056).
⚠️ Report is 2 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
beetsplug/ftintitle.py	87.50%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6732      +/-   ##
==========================================
+ Coverage   74.51%   74.53%   +0.02%     
==========================================
  Files         162      162              
  Lines       20822    20832      +10     
  Branches     3298     3298              
==========================================
+ Hits        15515    15527      +12     
+ Misses       4550     4549       -1     
+ Partials      757      756       -1     

Files with missing lines	Coverage Δ
beetsplug/ftintitle.py	91.05% <87.50%> (+2.56%)	⬆️

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[github-actions]

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.