Align backend plan with codebase

lib/server/responses.ts

@@ -0,0 +1,8 @@
+import { NextResponse } from "next/server";
+
+export function dbUnavailable(): NextResponse {
+  return NextResponse.json(
+    { error: "Database is not configured (DATABASE_URL)." },
+    { status: 503 },
+  );
+}