Fix stale featured event on homepage: revalidate cache when featured event changes

- Extract revalidateFrontendCache() to backend/src/lib/revalidate.ts
- Call revalidation from site-settings when featuredEventId is set/cleared
- Ensures homepage shows updated featured event after admin changes

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/src/lib/revalidate.ts

@@ -0,0 +1,22 @@
+// Trigger frontend cache revalidation (fire-and-forget)
+// Revalidates both the sitemap and the next-event data (homepage, llms.txt)
+export function revalidateFrontendCache() {
+  const frontendUrl = process.env.FRONTEND_URL || 'http://localhost:3002';
+  const secret = process.env.REVALIDATE_SECRET;
+  if (!secret) {
+    console.warn('REVALIDATE_SECRET not set, skipping frontend revalidation');
+    return;
+  }
+  fetch(`${frontendUrl}/api/revalidate`, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ secret, tag: ['events-sitemap', 'next-event'] }),
+  })
+    .then((res) => {
+      if (!res.ok) console.error('Frontend revalidation failed:', res.status);
+      else console.log('Frontend revalidation triggered (sitemap + next-event)');
+    })
+    .catch((err) => {
+      console.error('Frontend revalidation error:', err.message);
+    });
+}