fix(security): 전체 서비스 보안 점검 — XSS·인가·토큰·헤더·에러마스킹 일괄 수정

Phase 1 CRITICAL XSS:
- marked.parse() → DOMPurify.sanitize() (system3 ai-assistant, issues-management)
- toast innerHTML에 escapeHtml 적용 (system1 api-base, system3 common-header)
- onclick 핸들러 → data 속성 + addEventListener (system2 issue-detail)

Phase 2 HIGH 인가:
- getUserBalance 본인확인 추가 (tksupport vacationController)

Phase 3 HIGH 토큰+CSP:
- localStorage 토큰 저장 제거 — 쿠키 전용 (7개 서비스)
- unsafe-eval CSP 제거 (system1 security.js)

Phase 4 MEDIUM:
- nginx 보안 헤더 추가 (8개 서비스)
- 500 에러 메시지 마스킹 (5개 API)
- path traversal 방지 (system3 file_service.py)
- cookie fallback 데드코드 제거 (4개 auth.js)
- /login/form rate limiting 추가 (sso-auth)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

system3-nonconformance/api/services/file_service.py

@@ -129,7 +129,9 @@ def delete_file(filepath: str):
     try:
         if filepath and filepath.startswith("/uploads/"):
             filename = filepath.replace("/uploads/", "")
-            full_path = os.path.join(UPLOAD_DIR, filename)
+            full_path = os.path.normpath(os.path.join(UPLOAD_DIR, filename))
+            if not full_path.startswith(os.path.normpath(UPLOAD_DIR)):
+                raise ValueError("잘못된 파일 경로")
             if os.path.exists(full_path):
                 os.remove(full_path)
     except Exception as e: