Weekly AI digests for a collection

Returns AI-generated weekly "mini blog post" digests for the collection, newest-first, cursor-paginated. Rows omit body/releaseIds — fetch a single week via GET /v1/collections/:slug/digests/:weekStart for the full row. Digests are generated on ET Mondays for the just-closed week by the collection-summaries cron and only exist for weeks that cleared the quality floor (>=3 substantive releases).