-- 每个 run 只保留最新的增量 checkpoint；写入必须通过当前 worker lease fencing。
CREATE TABLE IF NOT EXISTS run_checkpoints (
    run_id              TEXT PRIMARY KEY REFERENCES runs(id) ON DELETE CASCADE,
    phase               TEXT NOT NULL,
    step                INTEGER NOT NULL,
    next_step           INTEGER NOT NULL,
    messages_json       TEXT NOT NULL,
    provider_request_id TEXT,
    tool_call_id        TEXT,
    attempt             INTEGER NOT NULL,
    updated_at          INTEGER NOT NULL
);