feat(anomaly detection): Cron to cleanup disabled alerts

[aayush-se]

• Currently, cleanup only happens upon detection call leaving stale timestamps for disabled alerts to accumulate
• Created cron which runs weekly to cleanup (delete) disabled alerts based on when they were last queued for detection (28 days) or if last_queued is null then deletes old alerts
• Cleaning alerts as a byproduct cleans the timeseries that are associated with them because of the parent-child relationship