- Add TimeSlotEdit schema (partial update, all fields optional)
- Add TimeSlotEditResponse schema
- Add PATCH /calendar/slots/{slot_id} for editing real slots
- Add PATCH /calendar/slots/virtual/{virtual_id} for editing virtual slots
- Triggers materialization before applying edits
- Detaches from plan after edit
- Both endpoints enforce past-slot immutability, overlap detection, plan
detachment, and workload warnings