Start Sync Chat Review

Ensure a post-scott sync review turn is in flight, and dispatch it.

Called by both the CLI (after an upload) and the /sync page (on an empty or stale thread), so it is idempotent: if a review turn is already pending or streaming at the thread tip, it re-nudges that turn instead of stacking a duplicate. A fresh review is only created once the previous one has settled, so a later sync reflects the newly uploaded sessions.

Dispatch is best-effort. The pending row is the durable work item; on a missed dispatch reconcile_sync_chat_turns re-dispatches it, so this returns 200 (the review is queued) rather than 503.