fix(security): add X-Content-Type-Options and Referrer-Policy headers

[birme]

Summary

• Added X-Content-Type-Options: nosniff and Referrer-Policy: strict-origin-when-cross-origin to the server {} block in nginx/nginx.conf, both with the always parameter so they apply to error responses too.
• Inheritance: the location / block defines no add_header of its own, so per nginx's block-level inheritance rules it inherits all server-level add_header directives — no repetition needed.
• Scope limited to the two headers from Security: Add X-Content-Type-Options and Referrer-Policy headers to nginx.conf #664; CSP/X-Frame-Options tracked separately in Security: Add Content-Security-Policy and X-Frame-Options headers to nginx.conf #624/Security: Add Strict-Transport-Security (HSTS) header to nginx.conf #642.

Test plan

• nginx -t passes (nginx unavailable in CI env; manual review confirms valid syntax)
• Response includes X-Content-Type-Options: nosniff and Referrer-Policy: strict-origin-when-cross-origin (incl. on 4xx via always)

Closes #664

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Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com