- New service: app/services/plan_slot.py
- Virtual slot ID: plan-{plan_id}-{YYYY-MM-DD} format with parse/make helpers
- Plan-date matching: on_month/on_week/on_day hierarchy with week_of_month calc
- Materialization: convert virtual slot to real TimeSlot row from plan template
- Detach: clear plan_id after edit/cancel to break plan association
- Bulk materialization: materialize_all_for_date for daily pre-compute
- New tests: tests/test_plan_slot.py (23 tests, all passing)
