Add support for AudioMuse-AI

Hi team,

I’d like to propose adding native support for [AudioMuse-AI](https://github.com/NeptuneHub/AudioMuse-AI)
as an optional backend for audio attribute extraction in Beets.

Beets users rely on smart playlists, recommendations, and other rule-based automations that depend on rich audio attributes. Today, many of us rely on Spotify’s `audio_features` for loudness, tempo, energy, valence, danceability, and similar attributes; however, the Spotify API is effectively deprecated for third-party use.

AudioMuse-AI provides a fully local solution that generates a comparable set of audio embeddings and attributes, eliminating the need for external APIs. It naturally fits into Beets’ philosophy of local, scriptable, and privacy-friendly music management.

I have a working integration that calls AudioMuse-AI locally, extracts track-level embeddings and attributes, stores them in Beets fields, and makes them available for queries, smart playlists, and rule-based organization.

Before polishing and submitting a PR, I wanted to check whether we are interested in adding this to the core or if I should keep it as a separate plugin.

cc: @snejus @semohr @JOJ0

No files, tests, or entry points are named. Review the existing working AudioMuse-AI integration and the proposal to determine whether support belongs in Beets core or a separate plugin; done means an agreed integration path before polishing or submitting a PR.

Written by the indexing model from the issue text.