Delete Session

Delete a session: it disappears from the session lists and can no longer be opened or continued.

Soft delete (mirrors V2's is_archived): the runs, events and usage records stay in place so billing and analytics keep reconciling, and ZDR purge is the only thing that ever destroys V4 content. Idempotent — deleting twice is a no-op 204.

An in-flight run is cancelled first, and pending queued messages are cancelled too; otherwise the drain would dispatch a follow-up run whose session_archived_at is NULL and resurrect the session. Cancel is forwarded to CP before anything is written (and outside the queue lock, per the enqueue path's ordering), so a CP failure (502) leaves the session visible rather than hiding a run that is still burning credits.