[API] Expose collection scheduling metadata needed to interpret card due dates

[lonewolf2208]

Problem

The cards API exposes RAW_DUE. For review cards, this is a collection scheduler day number, not a calendar date or Unix timestamp.

External API consumers cannot reliably convert review-card RAW_DUE into a concrete calendar date without collection scheduling metadata, such as the collection creation time and day cutoff.

Proposed follow-up

Expose the collection scheduling metadata needed to interpret review-card due values through the API/content provider

Related comment - #20675 (comment)

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