🛡️ Sentinel: [MEDIUM] Add Security Headers and strict CORS policy to Fastify backend

[seonghobae]

This PR adds @fastify/helmet to add standard HTTP response headers to help secure the application (e.g. X-Frame-Options, Content-Security-Policy), and @fastify/cors with origin: false to ensure we define an explicit policy for cross-origin requests, preventing malicious cross-site requests.

This fixes a medium priority security vulnerability where the Fastify server lacked these baseline security configurations.

🚨 Severity: MEDIUM
💡 Vulnerability: Missing security headers and explicit CORS policy.
🎯 Impact: An attacker could potentially perform cross-site scripting (XSS) or clickjacking attacks, or cross-site request forgery if an overly permissive CORS policy was accidentally assumed or if no headers were present to block malicious framing/content loading.
🔧 Fix: Added @fastify/helmet and @fastify/cors to the apps/api/src/http/server.ts entry point, ensuring the server includes secure default headers and rejects cross-origin requests.
✅ Verification: pnpm test runs cleanly and ensures no regressions were introduced. Note: required using fastify v4 compatible plugin versions (@fastify/helmet@11.1.1 and @fastify/cors@8.5.0).

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PR created automatically by Jules for task 16701418282331058696 started by @seonghobae

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⛔ Files ignored due to path filters (1)

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📒 Files selected for processing (3)

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• apps/api/package.json
• apps/api/src/http/server.ts

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[seonghobae]

통합 PR에서 유효 변경만 선별해 다시 다루고 있어 이 중복/잔여 PR은 정리합니다. #39 및 vibemafiaclub/vooster#34를 기준으로 후속 검토를 진행합니다.