One list of the user as a CSV or Parquet file

Every record of one list in [start, end), as one file streamed in a single response (Content-Disposition: attachment). The columns are the members of the list's JSON records, in the same order and with the same names and values: times in UTC, a missing value empty (CSV) or null (Parquet), metadata (and the daily sources) as JSON text. samples holds every quantity type unless type names one. The file is not paged: cursor and limit are a 400. A failure once the file has started cuts the connection, so a truncated file never looks complete. See docs/exports.md.