Servers that block bot user-agents return 403 even for live URLs.
On a 403 response, the checker now retries with a browser-like UA;
if the server then responds with 2xx/3xx, the link is counted as OK.
This eliminates false positives from bot-blocking (e.g. bergbauspuren-bredelar.de).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>