fix(previews): respect download priority for iframe previews

[kv-chiu]

Description / 描述

This PR updates the logic in getPreviews to ensure that Iframe previews properly respect the strictly enforced preview_download_by_default setting.

Previously, iframePreviews were unconditionally pushed to the high-priority res array. Now, they check the downloadPrior flag:

If downloadPrior is true: Iframe previews are added to the subsequent list (low priority).
If downloadPrior is false: They are added to the res list (high priority), as before.

Motivation and Context / 背景

Relates to
OpenList#992
OpenList#1449
#237

How Has This Been Tested? / 测试

Checklist / 检查清单

• I have read the CONTRIBUTING document.
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• I have formatted my code with go fmt or prettier.
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• I have requested review from relevant code authors using the "Request review" feature when applicable.
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• I have updated the repository accordingly (If it’s needed).
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  • OpenList-Frontend #XXXX
  • OpenList-Docs #XXXX