Stop Persisting Schedule Tick State

Stop writing per-tick state (lastScheduledTimestamp, nextScheduledTimestamp, lastRunTriggeredAt) on TaskSchedule and TaskScheduleInstance. The schedule engine now carries the previous fire time forward via the worker queue payload, eliminating ~270K dead-tuple-driven autovacuums per year on these hot tables and the associated IO:XactSync mini-spikes on the writer. Customer-facing payload.lastTimestamp semantics are unchanged.