Detail backfill (run 217) hit 5 consecutive HTTP 429s and aborted at 19 enriched
because the detail fetcher only had a same-session 429 short-retry (no reconnect).
429s are persistent: keep-alive reuses one backconnect exit IP across sequential
detail fetches -> avito rate-limits it. Mirror the merged SERP 429 fix (#1769):
widen short-retry 4->8 and add up to 3 ephemeral-session reconnects (fresh exit IP)
on 429 exhaustion before raising. Caller session never touched (as with 403 path).
