Adds defense-in-depth HTTP headers to the dev and prod Caddyfiles. CSP,
X-Frame-Options, Permissions-Policy, and Cache-Control: no-store apply
to the dashboard SPA (/admin/*) and admin API (/api/v1/admin/*). nosniff
and Referrer-Policy apply to every response (a sane default). HSTS is
unconditional in prod.
User-route responses (the catch-all `handle`) deliberately get NO CSP —
user scripts own their own response headers by design.
The CSP / X-Frame-Options / Cache-Control / Permissions-Policy headers
use the `?` operator ("set only if not already present") so application
responses with their own restrictive policy — notably the audit C-2
file-download handler, which sends a sandboxed CSP — win over Caddy's
defaults.
This downgrades H-A4 (dashboard token in localStorage) from acute to
defense-in-depth: with this CSP and no inline JS in the dashboard,
XSS-based token exfiltration is no longer a one-step exploit.
Validated with `caddy validate` (docker caddy:2-alpine) for both files;
also `caddy fmt --overwrite`'d.
Audit ref: security_audit/07_http_cors_csrf_xss.md (H07-03/04/05 +
M07-06/07/08/09).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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