Add art:true/false album query to detect embedded cover art

[CarolineKevnick]

Adds support for a new art:true / art:false album-level query that allows users to filter albums based on whether they contain embedded cover art in their audio files.

• Adds a new art boolean field to the Album model
• Detects embedded art using MediaFile.art during import
• Persists the field in the library database
• Registers a custom ArtQuery so users can run CLI queries
• Example usage: beet ls -a art:true

This feature improves the usability of beets for users who embed artwork directly into music files and want to better manage or detect it.

[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.

[wisp3rwind]

Hi, thanks for contributing to beets!

However, I see a few issues here:

• Please read & follow our development guide at https://github.com/beetbox/beets/blob/master/CONTRIBUTING.rst, this PR messes up our code style in many (also unrelated) files, making it impossible to review. Eventually, this should also have tests, but it might make sense to first discuss here whether the code seems like a sensible approach to the problem.
• I suspect this is motivated by Query to find albums with/without embedded cover art #2572? It'd be great to mention that: In this particular case, your pull requests give me the impression that this might be a class project (correct me if I'm wrong!) rather than improving beets for your actual usecase. On its own, I don't think that would be sufficient motivation for adding a feature that potentially nobody uses.