feat(cms): keep the admin panel out of the index

X-Robots-Tag rather than the robots.txt Disallow alone, for the same
reason the staging rule uses one: a Disallow blocks crawling, not
indexing, so a URL found from an external link can be indexed without
ever being fetched — and blocking the crawl means the noindex is never
seen. Both mechanisms are applied to /admin and /cms-api.

The existing CSP is frame-ancestors only, which restricts who may embed
the site rather than what a page may load, so it cannot break the panel.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017YmbBhr8s7GusjDE12hrZM

nextjs-app/__tests__/app/nextConfig.test.ts

@@ -47,3 +47,20 @@ describe('next.config.mjs', () => {
     expect(csp!.value).toContain('https://analytics.schoolcompare.co.uk');
   });
 });
+
+describe('admin surface', () => {
+  it('serves noindex on the admin panel and the CMS API', async () => {
+    // robots.txt disallows these too, but a Disallow only blocks crawling — a
+    // URL found from an external link can still be indexed without ever being
+    // fetched. This header is what actually keeps them out.
+    const headers = await headerRules();
+    for (const source of ['/admin/:path*', '/cms-api/:path*']) {
+      const rule = headers.find((entry) => entry.source === source);
+      expect(rule).toBeDefined();
+      expect(rule!.headers).toContainEqual({
+        key: 'X-Robots-Tag',
+        value: 'noindex, nofollow',
+      });
+    }
+  });
+});