[Phase 6] PR 6-1: Workshop 모델

## 개요
Expert::Course, CourseApplication, Certificate 모델

## 스키마
- expert_courses: level(0-3,12), batch_number, start_date, end_date, max_participants, status(enum: draft/open/in_progress/completed), location, price, time
- expert_course_applications: applicant_id(FK→users), course_id(FK), status(enum), payment_status, payment_amount, payment_key, unique(applicant_id, course_id)
- expert_certificates: user_id(FK), course_id(FK), application_id(FK), certificate_type(enum: completion/qualification), certificate_number(unique, 9W-C-YYYY-NQ-NNN)

## 완료 기준
- 워크숍 과정/신청/인증서 모델
- 인증서 번호 자동 생성

## 의존성
PR 4-1 (Study 모델)