List Fetch fields

Every field and preset available to POST /v1/fetch, with type, unit, description, interpretation hints, null meaning, source and citation. 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. Enrich and Search have their own field lists at GET /v1/enrich/fields and GET /v1/search/fields.