middleware: add security response headers (closes #98)

Adds a SecurityHeaders middleware and registers it globally, right after the request ID middleware, so every route gets the headers, including static files, /metrics and error responses.

It sets Strict-Transport-Security (one year, includeSubDomains), a Content-Security-Policy with default-src 'self', no scripts and frame-ancestors 'none', X-Frame-Options DENY, X-Content-Type-Options nosniff, Referrer-Policy no-referrer and a Permissions-Policy that turns every listed feature off.

HSTS is sent on every response, not only over TLS: the service runs behind a TLS-terminating proxy and REPO_POLICIES.md requires the application to send it. Referrer-Policy is stricter than the policy baseline because dashboard URLs can name internal hosts.

model: claude-opus-4-8 (implementation); claude-fable-5 (commit message)

internal/server/routes.go

@@ -21,6 +21,7 @@ func (s *Server) SetupRoutes() {
 	// Global middleware
 	s.router.Use(chimw.Recoverer)
 	s.router.Use(chimw.RequestID)
+	s.router.Use(s.mw.SecurityHeaders())
 	s.router.Use(s.mw.Logging())
 	s.router.Use(s.mw.CORS())
 	s.router.Use(chimw.Timeout(requestTimeout))