Submit Session Followup

HTTP twin of the session_followup socket handler, for detached runs.

Queues a user turn and wakes the drain loop without a socket. The synchronous 202 replaces the socket ack, so a detached client cannot lose an ack under burst the way a socket emit can. Dedupe lives inside persist_and_announce_queued_followup; a replayed call_id returns the original id without a second row.