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/next.config.mjs

@@ -88,6 +88,23 @@ const nextConfig = {
           },
         ],
       },
+      {
+        /*
+         * The admin panel and the CMS API must never be indexed.
+         *
+         * X-Robots-Tag, not just the robots.txt Disallow, for the same reason
+         * the staging rule above uses one: a Disallow blocks crawling, which
+         * is not indexing. A disallowed URL found from an external link can
+         * still be indexed without ever being fetched — and worse, blocking
+         * the crawl means the noindex is never seen.
+         */
+        source: '/admin/:path*',
+        headers: [{ key: 'X-Robots-Tag', value: 'noindex, nofollow' }],
+      },
+      {
+        source: '/cms-api/:path*',
+        headers: [{ key: 'X-Robots-Tag', value: 'noindex, nofollow' }],
+      },
       {
         source: '/:path*',
         headers: [