- PATCH /calendar/plans/{plan_id}: edit a recurring schedule plan
- Validates period-parameter hierarchy after merge
- Rejects edits to inactive (cancelled) plans
- Detaches future materialized slots so they keep old data
- Past materialized slots remain untouched
- POST /calendar/plans/{plan_id}/cancel: cancel (soft-delete) a plan
- Sets is_active=False
- Detaches future materialized slots (plan_id -> NULL)
- Preserves past materialized slots, returns their IDs
- Added SchedulePlanEdit and SchedulePlanCancelResponse schemas