Generate valid, current-spec schema.org JSON-LD from plain fields — the complement to /api/schema/audit. POST {type, fields}; returns correctly nested JSON-LD, a ready-to-embed <script> tag, and a self-audit. Types: Product, Review, Article, Recipe, VideoObject, LocalBusiness, Organization, BreadcrumbList. Current to 2026 Google guidance (interactionStatistic, Key Moments clips, etc).