🛡️ Sentinel: [SECURITY ENHANCEMENT] Add defense-in-depth headers for cross-origin isolation

[NickJLange]

🚨 Severity: ENHANCEMENT
💡 Vulnerability: Missing defense-in-depth headers for cross-origin isolation and automated HTTPS upgrades.
🎯 Impact: Without these headers, the site is theoretically more vulnerable to XS-Leaks, side-channel attacks, or mixed content degradation if a non-secure link is embedded.
🔧 Fix: Updated the static site configuration in render.yaml to include Cross-Origin-Opener-Policy, Cross-Origin-Resource-Policy, and the upgrade-insecure-requests directive in the CSP.
✅ Verification: Ran bun run build successfully to confirm the site still compiles and there are no syntax errors in the YAML config.

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PR created automatically by Jules for task 3123671297183291753 started by @NickJLange

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Summary by cubic

Adds defense‑in‑depth security headers to enable cross‑origin isolation and enforce HTTPS across the site. Updates the CSP with upgrade-insecure-requests and sets Cross-Origin-Opener-Policy: same-origin and Cross-Origin-Resource-Policy: same-site in render.yaml to reduce XS‑Leak and mixed‑content risks.

^{Written for commit 612b288. Summary will update on new commits.}

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[cubic-dev-ai]

No issues found across 1 file