Meta Sharing Debugger receives 403 while Facebook crawler gets 200

Project: smakfin-web
Domain: https://dev.smakfin.cz/blog/test-facebook-propojeni-01

Meta Sharing Debugger consistently reports HTTP 403.

Verified:
- robots.txt explicitly allows facebookexternalhit and Facebot
- curl with the complete Meta user agent returns HTTP 200:
  facebookexternalhit/1.1 (+http://www.facebook.com/externalhit_uatext.php)
- Open Graph metadata is server-rendered and valid
- Bot Protection is Off
- AI Bots are Allow
- Attack Mode is Off
- There are no custom Firewall rules, blocked IPs, or Firewall events

The Vercel AI support suggested a Custom WAF bypass for ASN 32934,
but the dashboard explicitly states that Custom Bypass Rules do not bypass
system mitigations. System Bypass Rules are unavailable on the Hobby plan.

Could Vercel investigate a possible platform-level DDoS mitigation false
positive affecting Meta crawler traffic for this domain?