Make next event visible to AI crawlers (SSR, JSON-LD, meta, llms.txt)

- SSR next event on homepage; pass initialEvent from server to avoid client-only content
- Add schema.org Event JSON-LD on homepage when next event exists
- Dynamic homepage metadata (description, OG, Twitter) with next event date
- Add dynamic /llms.txt route for AI-friendly plain-text event info
- Revalidation: support next-event tag; backend revalidates sitemap + next-event on event CUD
- Allow /llms.txt in robots.txt

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

frontend/src/app/robots.ts

@@ -15,6 +15,7 @@ export default function robots(): MetadataRoute.Robots {
           '/contact',
           '/faq',
           '/legal/*',
+          '/llms.txt',
         ],
         disallow: [
           '/admin',