Get a collection's detail page payload

Returns the collection's name/description plus its ordered members. members is the canonical mixed-kind list (orgs + products); orgs is the legacy org-only subset retained for back-compat. Orgs join through organizations_public and products through products_active, so soft-deleted / on_demand rows never leak via a collection. GitHub handles are picked deterministically from org_accounts so multi-handle orgs don't fan out the row.