prune_sessions and delete_session only handled direct children when
satisfying the parent_session_id FK constraint. Multi-level chains
(A -> B -> C) caused IntegrityError because deleting B while C still
referenced it was blocked by the FK.
Fix: NULL out parent_session_id for any session whose parent is about
to be deleted. This orphans children instead of cascade-deleting them,
which also respects the prune retention window — newer child sessions
are no longer deleted just because an ancestor is old.
Reported by Aaryan2304 in PR #6463.
a94099908a