Mock server — available scenarios:

  GET /ok                             200 OK — site is up
  GET /forbidden                      403 Forbidden → HEAD_FORBIDDEN
  GET /blocked                        451 Legal Block → LEGAL_BLOCK_451
  GET /redirect/<domain>              301 Redirect → OK_NEW_DOMAIN (rebrand)
  GET /not-found                      404 Not Found → NOT_FOUND
  GET /server-error                   503 Service Unavailable
  GET /cloudflare                     403 + cf-ray → CLOUDFLARE_FORBIDDEN
  GET /slow[?delay=N]                 Delay Ns then 200 → timeout
  GET /provider-error                 200 + x-error-description → provider DOWN
  GET /healthz                        Health check