- Add DayOfWeek and MonthOfYear enums for plan period parameters
- Add SchedulePlan model with at_time/on_day/on_week/on_month fields
- Add DB-level check constraints enforcing hierarchy:
on_month requires on_week, on_week requires on_day
- Add application-level @validates for on_week range (1-4),
on_month hierarchy, and estimated_duration (1-50)
- Add is_active flag for soft-delete (plan-cancel)
- Add bidirectional relationship between SchedulePlan and TimeSlot
- All existing tests pass (29/29)