fix(seo): noindex parameterised comparisons, keep bare /compare

25,193 schools make ~317 million pairs. The bare page stays indexable as the
landing page for the head term; the parameter space goes noindex, follow so
its outbound links still count.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015mWQnpye9F299NVRCCSRvj

nextjs-app/app/compare/page.tsx

@@ -5,6 +5,7 @@
 
 import { fetchComparison, fetchMetrics } from '@/lib/api';
 import { ComparisonView } from '@/components/ComparisonView';
+import { absoluteUrl } from '@/lib/site';
 import type { Metadata } from 'next';
 
 interface ComparePageProps {
@@ -14,13 +15,33 @@ interface ComparePageProps {
   }>;
 }
 
-export const metadata: Metadata = {
-  title: 'Compare Schools',
-  description:
-    'Compare schools in England side by side — Ofsted inspections, KS2 and GCSE results against the England average, admissions odds and school community.',
-  keywords:
-    'school comparison, compare schools, Ofsted comparison, school admissions, KS2 comparison, primary school performance',
-};
+/**
+ * Indexability depends on the query string, so this cannot be a static export.
+ *
+ * Bare /compare is the landing page for the "compare schools" head term and
+ * stays indexable. /compare?urns=… is an unbounded parameter space — 25,193
+ * schools make ~317 million pairs — so it goes noindex. It stays `follow` and
+ * keeps a canonical to the bare path, so the links out to each school page
+ * still count.
+ */
+export async function generateMetadata(
+  { searchParams }: ComparePageProps,
+): Promise<Metadata> {
+  const { urns } = await searchParams;
+
+  const base: Metadata = {
+    title: 'Compare Schools',
+    description:
+      'Compare schools in England side by side — Ofsted inspections, KS2 and GCSE results against the England average, admissions odds and school community.',
+    keywords:
+      'school comparison, compare schools, Ofsted comparison, school admissions, KS2 comparison, primary school performance',
+    alternates: { canonical: absoluteUrl('/compare') },
+  };
+
+  if (!urns) return base;
+
+  return { ...base, robots: { index: false, follow: true } };
+}
 
 // Dynamic via searchParams; remove force-dynamic so internal data fetches
 // can still use Next.js's per-call revalidate cache.