Add library.db.backup to backup_exclude in config.yaml to prevent unnecessary backup bloat

[Schlumperdix]

Hello Music Assistant team,

First of all, thank you for this fantastic project.

I would like to suggest adding library.db.backup to the backup_exclude list in the add-on's config.yaml.
Currently, the add-on already excludes certain files from Home Assistant backups:

`
backup_exclude:

• cache.db
• collage_images/*
• .cache/*
  `

However, library.db.backup (created during database migrations) is left out. As a result, this large recovery artifact—which can easily reach several hundred megabytes—is included in every single daily Home Assistant backup, even though the migration has already successfully completed. A database reset or normal operation does not clean up this backup file, leaving it to continuously bloat the user's HA backups.

Adding it to the exclusion list would permanently solve this for all users.

Special thanks go to user el-bakkaliMehdi, whose valuable analysis and technical insights in the community discussion helped pinpoint this exact root cause. For further context, please refer to that discussion here:
https://github.com/orgs/music-assistant/discussions/6332

Thank you for considering this improvement!