Reschedule Overdue Work

Unfinished required work remains auditable on its original date and is never auto-copied to tomorrow. Owner/admin selects a date that exists inside the same active plan and enters a reason. Call the audited reschedule boundary so immutable plan_day_id remains original while current_plan_day_id changes. Reject completed/cancelled tasks and tasks with active timers. Re-evaluate destination-day workload and warn if unusually heavy, but never silently move additional tasks. Student cannot reschedule. Add idempotency, audit and RBAC tests.