system/backend/whistlegraph-turns.mjs is the Mongo queue from TURNS.md: enqueue (one row per requestID, three waiting per owner), claim (oldest first, one running turn per thread, a lease that went quiet for 90 s is taken over first, checkpoint carried across), heartbeat, complete and fail (lease holder only), read, listOpen, recent, depth. publicTurn never exposes the owner's subject. POST/GET /api/whistlegraph-turn submits for the signed-in owner against a thread they own, refusing a request that builds on a version the thread has moved past. Workers never use the route. Tested against a fake collection.