feat: ERD 도구 테이블/컬럼/외래키 삭제 기능 추가

[seonghobae]

변경 사항

ERD 모델 도구(packages/web/src/lib/erd.ts)에 사용자가 엔티티 관계를 관리하기 위해 필수적인 삭제 기능 3가지를 추가했습니다:

• removeTable(name: string): 테이블과 참조하는 외래키 제거
• removeColumn(tableName: string, columnName: string): 컬럼 제거 (외래키로 참조 중일 경우 삭제 방어)
• removeForeignKey(tableName: string, fkColumnName: string): 외래키 제약조건 제거

영향도 및 확인

• 100% 테스트 커버리지 유지 (packages/web/src/lib/erd.test.ts)
• 프론트엔드 기능에 영향 없음
• 보안 또는 기타 회귀 문제 없음

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PR created automatically by Jules for task 5568272533307663155 started by @seonghobae

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