Fetch structured data by URL or ASIN with optional field filters. Pass either url (an Amazon product page URL) or asin (10-char alphanumeric). fields (optional enum): profile | price | trend | ratings | competitiveness | seller | facts_check | red_flags | sentiment | data_limitations. live (optional, default false): when true, additionally hits a live data endpoint (~15-30s) and appends a "live" block with the freshest price, stock, Buy Box seller, rating, and review count, plus data_as_of and is_stale flags. Unknown field values return an error with the full list of allowed fields.