feat: add security response headers middleware (closes #98)

Add SecurityHeaders() to internal/middleware and register it in the
global middleware stack so every response - dashboard, embedded static
assets, healthchecks, JSON API, and metrics - carries the six response
headers required by REPO_POLICIES.md before tagging 1.0:

  Strict-Transport-Security: max-age=31536000; includeSubDomains
  Content-Security-Policy:   default-src 'self'; script-src 'none';
                             style-src 'self'; img-src 'self';
                             font-src 'none'; connect-src 'none';
                             object-src 'none'; base-uri 'none';
                             form-action 'none'; frame-ancestors 'none'
  X-Frame-Options:           DENY
  X-Content-Type-Options:    nosniff
  Referrer-Policy:           no-referrer
  Permissions-Policy:        unused browser features denied

The dashboard template ships no JavaScript, no inline styles, no inline
event handlers and no images, and its only subresource is the embedded
stylesheet at /s/css/tailwind.min.css, so the policy needs neither
unsafe-inline nor unsafe-eval. frame-ancestors 'none' is the primary
anti-framing control with X-Frame-Options as the legacy fallback.

HSTS is emitted unconditionally rather than gated on r.TLS, because the
service runs behind a TLS-terminating proxy and the browser must still
enforce HTTPS end to end.

The headers are set before the request reaches the next handler, so
they are present on error responses too, including recovered panics and
request timeouts.

Tests cover each header's exact value, the CSP's required and forbidden
directives, presence on a 500 response, and a render of the real
dashboard through the middleware confirming the page still references
its stylesheet.

TODO.md

@@ -92,6 +92,16 @@ Rationale, Design, TODO, License, Author) if any are still missing.
   `make check` into `script/lint`. `golangci-lint config verify` is
   deliberately omitted: it fetches its schema over an unpinned live
   HTTPS call
+- 2026-08-09: security response headers middleware
+  (`SecurityHeaders()` in `internal/middleware/middleware.go`)
+  registered globally in `internal/server/routes.go`, so HSTS, CSP,
+  `X-Frame-Options`, `X-Content-Type-Options`, `Referrer-Policy`, and
+  `Permissions-Policy` are set on every response including `/s/...` and
+  `/metrics`; the CSP needs no `unsafe-inline`/`unsafe-eval` because the
+  dashboard ships no JavaScript and no inline styles; HSTS is emitted
+  unconditionally per policy (TLS-terminating proxy in front). Remaining
+  1.0 hardening items — `http.Server` timeouts, request body limits,
+  rate limiting, CORS scoping — are tracked separately
 - 2026-08-07: golangci-lint bumped to v2.12.2 (commit-pinned installs
   in `Dockerfile` and `script/bootstrap`); `.golangci.yml` set to the
   org-standard v2-schema config used across the org's repos