Generate and validate cron expressions for scheduled tasks
Format: minute hour day-of-month month day-of-week
* (asterisk): Every value (e.g., every minute, every hour)
*/5: Every 5th value (e.g., */5 in minute = every 5 minutes)
1-5: Range (e.g., 1-5 in hour = 1am through 5am)
1,3,5: List (e.g., 1,3,5 in day-of-week = Monday, Wednesday, Friday)