-- 2026-05-18 PR-Worker-Pool-Registry-1A: worker_heartbeats latest-row 조회 인덱스.
-- alive 판정 SQL: SELECT ... ORDER BY heartbeat_at DESC LIMIT 1 per worker_id.
CREATE INDEX IF NOT EXISTS idx_worker_heartbeats_worker_at
    ON worker_heartbeats (worker_id, heartbeat_at DESC);