published should reflect the source page date, not the scan date

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Title: `published` should reflect the source page date, not the scan date

The current frontmatter `published` field appears to use the scan/import date when the page does not provide a published date. I think this is misleading.

My expectation is:
- `published` should represent the publication date found on the source page.
- If the source page does not provide a publication date, `published` should be left empty or clearly documented as a fallback.
- The scan/import date should be stored in a separate field such as `scanned`, `importedAt`, or similar.

This distinction matters because `published` is often used as the article date, while the scan date represents when the note was created locally.

If the current behavior is intentional, I think it should be documented clearly, because the field name suggests source metadata rather than local import metadata.