Request an async encrypted conversation/session export

POST body for an async encrypted conversation/session export.

Every optional field below is a narrowing filter with the SAME meaning and the SAME implementation as the matching GET /v1/conversations query param — both go through src/conversation/session_filters.py, so an export cannot select a different set of sessions than the list the operator was looking at. The export is channel-agnostic by default; channel just narrows it. [start, end) is always required and is not a "filter" in this sense — it bounds the job's size.

Status view of an export job (with per-file download tokens when ready).