PR 6-1: Workshop(Expert) 모델 구현

전문가 과정 모델 계층 구현. TDD 기반.

## 스키마 (티켓 명세 기준)
### expert_courses
- level:integer(0-3,12), batch_number:integer
- start_date:date, end_date:date
- max_participants:integer, status: enum (draft/open/in_progress/completed)
- location:string, price:integer, time:string
- title:string, slug:string(unique), description:text

### expert_course_applications
- applicant_id(FK→users), course_id(FK)
- status: enum (pending/approved/rejected/cancelled)
- payment_status:string, payment_amount:integer, payment_key:string
- unique index: [applicant_id, course_id]

### expert_certificates
- user_id(FK), course_id(FK), application_id(FK)
- certificate_type: enum (completion/qualification)
- certificate_number:string (unique, 형식: 9W-C-YYYY-NQ-NNN)
- issued_at:datetime, expires_at:datetime(nullable)

## 주의사항
- Expert 모듈 table_name_prefix "expert_", UUID PK
- inverse_of 필수, 슬러그 한국어 fallback
- certificate_number 자동 생성 콜백
- fixtures에 _fixture: model_class: