Export selected products and variants as CSV

Exports the given Master Product / Master Variant ids as a CSV. Reads product documents, custom-attribute schema, and linked assets from the database. Scope is recordIds only — there is no search-query export. Media cells use preview URLs joined by ;. Toriut Record ID and SKU are always included. Parent SKU, Toriut Parent Record ID, and Product Option columns are included only when the selection contains at least one Master Variant. A non-empty fields array is validated against the export-time schema; unrecognized keys return 400 and no CSV.