Adds `Permissions-Policy: camera=(), microphone=(), geolocation=()` to
the shared (security_headers) snippet, so both archiv vhosts and the
git vhost deny browser APIs the app does not use. Reduces blast radius
of an XSS landing in a privileged origin.
The deploy smoke steps in nightly.yml and release.yml gain a matching
assertion against the canonical header value, so a future Caddyfile
edit that drops or loosens the header (e.g. `camera=(self)`) fails the
deploy instead of regressing silently.
`caddy validate` against caddy:2 passes; both workflow YAMLs parse.
Addresses @nora's round-2 suggestion on PR #499 — "lower-impact than
CSP but nearly free".
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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