🛡️ Sentinel: [MEDIUM] Add HTTP security headers middleware

[bobdivx]

🛡️ Sentinel: [MEDIUM] Add HTTP security headers middleware

🚨 Severity: MEDIUM
💡 Vulnerability: The Axum backend lacked essential HTTP security headers (Strict-Transport-Security, X-Frame-Options, X-Content-Type-Options) exposing the application to clickjacking and MIME-sniffing risks.
🎯 Impact: Attackers could potentially embed the app in iframes or execute content misinterpretations.
🔧 Fix: Implemented an Axum middleware layer (security_headers) to automatically set DENY, nosniff, and HSTS defaults globally across endpoints.
✅ Verification: Review apps/server/src/security.rs and the router layer integration in apps/server/src/main.rs. All tests pass.

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PR created automatically by Jules for task 9086806996185236049 started by @bobdivx

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