audiobookshelf: use from_utc_timestamp helper for ms-epoch conversions

[OzGav]

What does this implement/fix?

As requested here #3525 (comment) Marcel wanted a helper added but it actually already exists we just have to use it!

Related issue (if applicable):

• related issue

Types of changes

• 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

• The code change is tested and works locally.
• pre-commit run --all-files passes.
• 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.
• I have read and complied with the project's AI Policy for any AI-assisted contributions.

[MarvinSchenkel]

Automated review (no CRITICAL or PROBLEM findings): clean refactor that swaps two datetime.fromtimestamp(..., tz=UTC) call sites in the audiobookshelf parsers for the existing from_utc_timestamp helper. Behavior is identical, all call sites covered, imports cleaned up correctly, CI green.

Thanks @OzGav 🎉