Stream Kg Events

Server-Sent Events stream of `kg.session.committed` / `kg.board.cleared` events for the given board. The stream reads the `global_update_outbox` table for the latest events and emits new rows at a 1-second poll cadence. This endpoint is the backing data source for the React `useKgLiveEvents` hook (Frontend responsibility — hook implementation lives in the sibling UI repo). Protocol: event: kg.session.committed data: {"event_id": ..., "session_id": ..., "payload": {...}} Filter by `since` to resume a dropped connection without replaying the entire outbox backlog. Absence ⇒ start from now(). Bug fix (QueuePool exhaustion): este handler NÃO injeta mais ``db: AsyncSession = Depends(get_db)`` porque a session da request ficaria viva durante toda a vida do SSE stream (loop ``while True`` forever) — cada cliente prendia 1 conexão do pool, e um pool de 15 saturava com poucas tabs/clientes (somando consolidation_worker, outbox_worker, dispatcher e endpoints normais). Em vez disso abrimos uma sessão **por iteração** via ``async_session_factory``, devolvendo a conexão ao pool entre os ``sleep(1.0)``.