Make DNS servers fully dynamic from config.json

DNS server IDs (dns1, dns2, dns3) were hardcoded throughout the frontend
and backend. Now config.json's dnsServers object is the single source of
truth — adding or removing a DNS server in config automatically updates
the dashboard cards, credential modal, health checks, and probes.

- credentials.js: rebuild modal sections dynamically from SITE.dnsServers
- globals.js: add getPrimaryDnsId() helper for primary DNS lookups
- service-create.js, service-infrastructure.js: use dynamic DNS ID
- startup-validator.js: dynamic topCardServices from config
- middleware.js: add license endpoints to public routes

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

dashcaddy-api/middleware.js

@@ -293,6 +293,8 @@ module.exports = function configureMiddleware(app, {
     { path: '/api/logo',              exact: true,  method: 'GET' },
     { path: '/api/favicon',           exact: true,  method: 'GET' },
     { path: '/api/themes',            exact: true,  method: 'GET' },
+    { path: '/api/license/status',    exact: true,  method: 'GET' },
+    { path: '/api/license/feature/',  prefix: true, method: 'GET' },
   ];
 
   function isPublicRoute(req) {