Upsert a product

Create a new product or update the existing one whose externalId matches the body. Embeddings for the product's searchable text are generated and persisted as part of the call, so the product becomes searchable immediately after a successful response. Required fields: externalId, name. Optional: description, price (in cents), priceMin / priceMax, stockStatus (in_stock | out_of_stock | limited), images (array of URLs), brand, category, attributes (key/value). Workspaces with native Shopify / WooCommerce sync should normally rely on the plugin rather than calling this endpoint directly. Idempotent on externalId.