In-memory AdmissionController (idle/reserved/running, FIFO tickets keyed by
the session cookie, reservation and heartbeat expiry, promotion after
cleanup, EMA wait estimate) behind POST /api/v1/queue/join,
GET /api/v1/queue/status and DELETE /api/v1/queue/ticket. POST /api/v1/query
claims the session's reservation first and joins implicitly when idle.
Settings: CONFLUENCE_WEB_QUEUE_{RESERVATION_SECONDS,HEARTBEAT_SECONDS,MAX_LENGTH}.
