Add support for creating indices on arbitrary database tables

[semohr]

Creating indexes turned out to be relatively straightforward!
While we can’t remove them yet, that doesn’t seem necessary for now. Interestingly, much of the infrastructure for database additions was already in place.

• This PR introduces a new type, Index, which can be used to create an index on any defined table. 🎉
• The items table now automatically registers an index on album_id

Closes #5809

[Copilot]

Pull Request Overview

This PR adds support for creating database indices on arbitrary tables to improve query performance. The implementation introduces a new Index type and applies it to create an index on the album_id column of the items table.

• Introduces a new Index type for defining database indices
• Adds infrastructure to automatically create indices during table setup
• Creates an index on album_id for the items table to speed up album-based queries

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
docs/changelog.rst	Documents the new index creation for album_id queries
beets/library/models.py	Adds index definition to the Item model
beets/dbcore/types.py	Defines the new Index type
beets/dbcore/db.py	Implements index creation logic in database setup