Unify URL resolution, add health checker sync, and make modules optional

- Add url-resolver.js with single resolveServiceUrl() used by all 5 consumers
  (probes, health routes, health checker auto-config)
- Health checker now does full sync (add/update/remove) instead of add-only,
  and re-syncs automatically after every services.json mutation
- docker-maintenance and log-digest are now optional imports with try/catch,
  preventing container crashes when these files are absent
- Add null guards in routes/logs.js for graceful 503 responses

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

dashcaddy-api/url-resolver.js

@@ -0,0 +1,32 @@
+/**
+ * Unified URL Resolver
+ * Single source of truth for resolving service URLs across all systems
+ * (probes, health checks, health checker auto-config, SSO).
+ */
+
+/**
+ * Resolve the canonical URL for a service.
+ *
+ * Priority:
+ *   1. internet → https://www.google.com
+ *   2. isExternal + externalUrl → use as-is
+ *   3. service.url → prepend https:// if no protocol
+ *   4. dnsServers config → http://{ip}:{port}
+ *   5. fallback → buildServiceUrl(id)
+ *
+ * @param {string} id - service identifier
+ * @param {Object|null} service - service object from services.json (may be null for top-card services)
+ * @param {Object|null} siteConfig - site config containing dnsServers etc.
+ * @param {Function} buildServiceUrl - fallback URL builder (subdomain or subdirectory mode)
+ * @returns {string} resolved URL
+ */
+function resolveServiceUrl(id, service, siteConfig, buildServiceUrl) {
+  if (id === 'internet') return 'https://www.google.com';
+  if (service?.isExternal && service.externalUrl) return service.externalUrl;
+  if (service?.url) return service.url.startsWith('http') ? service.url : `https://${service.url}`;
+  const dnsServer = siteConfig?.dnsServers?.[id];
+  if (dnsServer) return `http://${dnsServer.ip}:${dnsServer.port || 5380}`;
+  return buildServiceUrl(id);
+}
+
+module.exports = { resolveServiceUrl };