feat: auto-update sitemap when events are added/updated/removed

- Use tag-based cache for sitemap event list (events-sitemap)
- Add POST /api/revalidate endpoint (secret-protected) to trigger revalidation
- Backend calls revalidation after event create/update/delete
- Add REVALIDATE_SECRET to .env.example (frontend + backend)

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

frontend/src/app/sitemap.ts

@@ -12,7 +12,7 @@ interface Event {
 async function getPublishedEvents(): Promise<Event[]> {
   try {
     const response = await fetch(`${apiUrl}/api/events?status=published`, {
-      next: { revalidate: 3600 }, // Cache for 1 hour
+      next: { tags: ['events-sitemap'] },
     });
     if (!response.ok) return [];
     const data = await response.json();