Add version update check feature for admin users

Checks current version against latest GitHub release every 6 hours via
cron job, caches result in appSettings, and surfaces updates through a
toast notification (once per browser session) and an animated badge in
the settings footer. Includes an admin toggle to enable/disable checks
(enabled by default). Also renames ServerSection to RegistrationSection
now that update checks are in their own card.

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

app/(pages)/settings/_components/system-health-section.tsx

@@ -27,6 +27,7 @@ const JOB_LABELS: Record<string, string> = {
   refreshTvChildren: "TV episodes",
   cacheImages: "Image cache",
   scheduledBackup: "Backup",
+  updateCheck: "Update check",
 };
 
 function formatBytes(bytes: number): string {