Fix Audible podcast sync skipping Periodical series (#5907)

# What does this implement/fix?

<!-- Quick description and explanation of changes. -->

The library sync filters items by `content_delivery_type` and only
accepted `PodcastParent`. Older Audible Original series report
`Periodical` instead, so they were dropped before the podcast import ran
and never appeared in the library. Nothing was logged, so the sync
looked like it had finished cleanly.

`Periodical` parents hold their episodes the same way `PodcastParent`
ones do, so no other changes were needed.

**Related issue (if applicable):**

- related issue https://github.com/music-assistant/support/issues/6077

## Types of changes

<!--
Tick exactly one box. CI (.github/workflows/pr-labels.yaml) derives
the label from the ticked box and applies it automatically; the
release-notes generator uses that same label to slot this change
into the next release notes.
-->

- [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: Claude <[email protected]>