- Add TimeSlotCreate, TimeSlotResponse, TimeSlotCreateResponse schemas
- Add SlotConflictItem, SlotTypeEnum, EventTypeEnum, SlotStatusEnum to schemas
- Add POST /calendar/slots endpoint with:
- Overlap detection (409 on conflict)
- Workload warning computation (advisory, returned in response)
- Support for event_type + event_data (job/system/entertainment)
- Default date to today if not provided
- Add _slot_to_response helper for ORM -> schema conversion