[FEAT] s3 삭제 건너뛰기 구현

[taerimiiii]

🔗 Related Issue

• resolve [FEAT] S3 삭제 건너뛰기 추가 #297

✨ 작업 개요

작업 내용을 간략하게 작성해주세요.

• S3Utils.java -> RESERVED_PREFIXES = {"festival", "contest"} 상수 + 두 메서드 추가
• isReservedKey(String key) — key의 첫 번째 경로 세그먼트가 reserved인지 확인
• deleteIfNotReserved(String key) — reserved면 debug 로그 후 스킵, 아니면 deleteFile() 호출 (실패 시 warn 로그만 남기고 예외 전파 안 함)

Reserved key 동작

• festival/banner.jpg → 스킵 (debug 로그)
• contest/2024/img.png → 스킵 (debug 로그)
• pins/uuid-photo.jpg → 정상 삭제
• solver-photos/uuid.jpg → 정상 삭제

체크리스트

• Reviewers, Assignees, Labels를 모두 등록했나요?
• .gitignore 설정을 하였나요?
• PR 머지 전 반드시 CI가 정상적으로 작동하는지 확인해주세요!

📷 이미지 첨부 (선택)

• 작업 결과를 확인할 수 있는 이미지나 GIF를 첨부해주세요.
• UI 변경, API 응답 샘플, 테스트 결과 등이 포함되면 좋아요!

🧐 집중 리뷰 요청

리뷰어가 특별히 봐주었으면 하는 부분이 있다면 작성해주세요.

[gemini-code-assist]

Code Review

This pull request introduces automatic S3 object cleanup when deleting pins, communities, problem solvers, or users. Before deleting database records, S3 keys are collected and subsequently deleted using a new S3Utils.deleteIfNotReserved utility, which skips reserved system assets. The review feedback highlights a critical improvement opportunity: S3 deletions currently run within active database transactions, which can cause data inconsistency if a transaction rolls back. To resolve this, it is recommended to defer S3 deletions until after a successful commit using TransactionSynchronizationManager. Additionally, feedback suggests refining the reserved key detection in S3Utils to handle leading slashes or spaces safely, and renaming a repository method to maintain consistent naming conventions.

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