Export Documents Csv

CSV export of the FULL filtered/sorted result set -- not the visible page (D4). Same filters as GET /api/documents (status, recipient, date_from, date_to, q, sort, direction; no offset/limit -- there is no page here) and the same ownership scope (SigningSession.org_id == ctx.org_id, applied inside _base_query) -- a caller can never export another org's envelopes, the same as they can never search or list them.

Bounded at _MAX_EXPORT_ROWS: exceeding it is a 413 naming the actual total and the limit, so the caller can narrow with a filter and retry, rather than the request hanging until something upstream times it out.