Fix filesystem scan crash on non-decimal digits in names (#6102)

# What does this implement/fix?

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`str.isdigit()` is True for characters like the superscript `³`, but
`int()` only accepts decimal digits, so a folder such as "Cherry Moon -
The Compilation 2002³" made the natural sort key raise `ValueError:
invalid literal for int() with base 10: '³'`, failing the scan of that
directory on every sync.

Use `isdecimal()`, which matches exactly the set of characters `\d`
splits on and `int()` accepts.

**Related issue (if applicable):**

- related issue https://github.com/music-assistant/support/issues/6245

## Types of changes

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- [X] Bugfix (non-breaking change which fixes an issue) — `bugfix`
- [ ] New feature (non-breaking change which adds functionality) —
`new-feature`
- [ ] Enhancement to an existing feature — `enhancement`
- [ ] New music/player/metadata/plugin provider — `new-provider`
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected) — `breaking-change`
- [ ] Refactor (no behaviour change) — `refactor`
- [ ] Documentation only — `documentation`
- [ ] Maintenance / chore — `maintenance`
- [ ] CI / workflow change — `ci`
- [ ] Dependencies bump — `dependencies`

## Checklist

- [X] The code change is tested and works locally.
- [X] `pre-commit run --all-files` passes.
- [X] `pytest` passes, and tests have been added/updated under `tests/`
where applicable.
- [ ] For changes to shared models, the companion PR in
`music-assistant/models` is linked.
- [ ] For changes affecting the UI, the companion PR in
`music-assistant/frontend` is linked.
- [X] I have read and complied with the project's [AI
Policy](https://github.com/music-assistant/.github/blob/main/AI_POLICY.md)
for any AI-assisted contributions.
- [ ] I have [raised a PR against the documentation
repository](https://github.com/music-assistant/music-assistant.io/blob/main/CONTRIBUTING.md)
targeting the main or beta branch as appropriate.

Co-authored-by: Marvin Schenkel <[email protected]>