A small, safe set applied to every response via middleware: X-Content-Type-Options
nosniff, Referrer-Policy strict-origin-when-cross-origin, CSP frame-ancestors
'none' (anti-clickjacking; unrelated to the YouTube/Plex embeds we host), and
Permissions-Policy disabling camera/mic/geolocation. A full CSP and HSTS are
deferred (HSTS belongs on the Caddy edge).