🛡️ Sentinel: [보안 개선] HSTS(Strict-Transport-Security) 헤더 추가

[seonghobae]

ViewerSecurityHeadersWebFilter에서 CSP(Content Security Policy) 등은 잘 적용되어 있으나 다운그레이드 공격을 방지하기 위한 HSTS(Strict-Transport-Security) 헤더가 누락되어 있었습니다. 이를 추가하고, 100% 테스트 커버리지를 만족하기 위해 해당 헤더 검증 테스트를 업데이트했습니다. Checkstyle 검사도 모두 통과합니다.

---

PR created automatically by Jules for task 15935699549286334287 started by @seonghobae

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.

[seonghobae]

중복 정리(2차): '전역 보안 헤더(HSTS/nosniff)' 클러스터 중복 → 정본 #56 유지. 동일 목적/파일로 동시 병합 불가. 고유 변경은 #56에 반영하거나 재오픈.

[google-labs-jules]

중복 정리(2차): '전역 보안 헤더(HSTS/nosniff)' 클러스터 중복 → 정본 #56 유지. 동일 목적/파일로 동시 병합 불가. 고유 변경은 #56에 반영하거나 재오픈.

Understood. Acknowledging that this work is now obsolete and stopping work on this task.