List Enrich fields and presets

Every field ID and preset accepted by POST /v1/enrich, with type, unit, description, interpretation hints, null meaning, source and preset membership, plus the Enrich price. Use it to choose a small fields or preset selection before enriching a parcel. No authentication is required. The response carries an ETag and Cache-Control: public, max-age=3600; send If-None-Match to receive 304 when nothing changed.