feat: add update-available detection

app/api/system/update-status/route.ts

@@ -0,0 +1,22 @@
+import { NextResponse } from 'next/server';
+import { loadState } from '@/lib/version-check';
+
+// Public endpoint that returns the latest cached update status. Fed by the
+// background scheduler started in instrumentation.node.ts; we never trigger
+// a fresh upstream fetch from this route, so an unauthenticated client cannot
+// use it to amplify traffic to the version server.
+export async function GET() {
+  const state = await loadState();
+  return NextResponse.json(
+    {
+      status: state.status,
+      lastCheckedAt: state.lastCheckedAt,
+      lastSuccessAt: state.lastSuccessAt,
+    },
+    {
+      headers: {
+        'Cache-Control': 'no-store',
+      },
+    },
+  );
+}