List curated collections

Returns every collection with a member count and a small previewMembers array (capped at 3) so the list page can render inline avatars without a second round trip. previewMembers is the mixed-kind preview (orgs and products); previewOrgs is the legacy org-only preview retained for back-compat. Member counts and previews are joined through organizations_public (for orgs) and products_active (for products), so soft-deleted / on_demand rows never inflate the totals. Pass ?featured=1 to return only homepage-featured collections.