🛡️ Sentinel: [security improvement] Add Security Headers Middleware

[CFMVCarlos]

🛡️ Sentinel: [security improvement] Add Security Headers Middleware

🚨 Severity: MEDIUM
💡 Vulnerability: Missing security headers (MIME sniffing and Clickjacking risks)
🎯 Impact: Without these headers, the application is vulnerable to MIME type confusion attacks and Clickjacking, where an attacker could potentially embed the application in a malicious frame.
🔧 Fix: Added a FastAPI middleware in app/main.py that appends X-Content-Type-Options: nosniff, X-Frame-Options: DENY, and Strict-Transport-Security: max-age=31536000; includeSubDomains to all HTTP responses.
✅ Verification: A new test test_security_headers has been added to test_app.py to ensure these headers are correctly applied to the responses. The entire test suite was successfully executed using python3 -m pytest.

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PR created automatically by Jules for task 9140810118626821264 started by @CFMVCarlos

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