Autoplay audio labels on page flip

Blocked by https://github.com/getodk/web-forms/issues/254

### Description
Implement support for the `autoplay` column so that audio or video media attached to a question automatically plays when the question becomes visible (after pagination/page transition).

[Specs](https://docs.getodk.org/form-styling/#autoplaying-media)

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### Checklist
- **Does it need UI/UX design?**
- [ ] Yes. Describe the UI/UX requirements or attach a Figma link in the "Design" section.
- [x] No
- **Does it need API design?**
- [x] Yes. Specify the endpoints, methods, or data structures needed.
- [ ] No
- **Does it need design around state flow?**
- [ ] Yes. Outline the state flow requirements.
- [x] No
- **Does it need a test plan?**
- [x] Yes. Add the test plan as a comment in this task or attach the document.
- [ ] No
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### User Stories

As a form designer / enumerator,
I want audio and video media to play automatically when a question appears (like in ODK Collect),
So that respondents don't have to manually press play, improving usability and reducing errors in guided interviews.

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### Design

- **Figma**: [Insert URL]
- **Design Docs**: [Insert URL]

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### Dependencies

- Pagination / question navigation must be implemented
- https://github.com/getodk/web-forms/issues/608
- https://github.com/getodk/web-forms/issues/607
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### Additional Notes

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### Related Issues

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Start by reviewing the blocker #254 and dependencies #608 and #607, then read the linked ODK autoplay specification. Confirm the pagination and question-navigation behavior before implementing automatic playback for media when a question becomes visible, with tests covering page transitions and the documented autoplay cases.

Written by the indexing model from the issue text.