[FEATURE] Allow disabling jitter delay on recurring scheduled tasks

Currently, all recurring scheduled tasks have a hardcoded deterministic jitter of up to ~6 minutes added at dispatch time to balance server load. There is no parameter to opt out.

For time-sensitive workflows (e.g., email triage sessions that should fire at exactly 9:00 AM, 1:00 PM, and 6:00 PM), even a few minutes of drift matters — especially when coordinating with a user's daily routine.

Request: Add an optional disableJitter: true parameter to create_scheduled_task and update_scheduled_task that fires the task at the exact cron time with no delay.