fetch_detail raised on the first 403/429, aborting the detail backfill (run 191
enriched only 48 then zombied) -> avito detail coverage stuck at 14% of active.
Mirror the merged SERP fix (#1765): on 403/firewall under the backconnect proxy,
rebuild an ephemeral session (fresh exit IP) and retry up to N before raising;
short-retry 429. Never close/rebuild the caller-provided shared session. Also
add the missing HTTP-200 firewall-interstitial check on the curl detail path.