devops(caddy): apply full security_headers snippet to GlitchTip vhost

The GlitchTip vhost only had a manual HSTS header; the rest of the
(security_headers) snippet (X-Content-Type-Options, Referrer-Policy,
Permissions-Policy, -Server removal) was missing.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

infra/caddy/Caddyfile

@@ -95,6 +95,6 @@ grafana.archiv.raddatz.cloud {
 }
 
 glitchtip.archiv.raddatz.cloud {
-	header Strict-Transport-Security "max-age=31536000; includeSubDomains; preload"
+	import security_headers
 	reverse_proxy 127.0.0.1:3002
 }