Under a backconnect rotating proxy (no changeip URL) a single HTTP 403 or
firewall interstitial aborted the entire avito_full_load sweep via
AvitoBlockedError -> mark_banned. Each new connection through backconnect
gets a fresh exit IP, so retry by rebuilding the curl_cffi session up to
_AVITO_403_MAX_RETRIES times before giving up. Legacy single-IP changeip
rotation and browser-mode paths unchanged. Mirrors existing 429 short-retry.