Arbitrage signal history for one event

Per-event detected arbitrage opportunities — newest first. Each row records the legs compared, the implied edge in basis points, and the (provider, market_id) pair where the price discrepancy was observed. Use to drive an arbitrage alert UI or to backtest signal quality. `limit` clamped to [1, 1000]; default 100. No time-window parameter — rows are returned in `recorded_at DESC` order.