avito_detail_backfill (mode=browser) aborted prod run 458 with
attempted=5 enriched=0 blocked=5: the lat-null pending queue is dominated
by DEAD listings (404 — that's why they are stale coord-holes). In browser
mode a dead listing's 404 page ("Ошибка 404. Страница не найдена") has no
item-view, so _is_detail_soft_block returned True → AvitoBlockedError →
backfill counted it as `blocked` and the consecutive-block breaker aborted
the run before reaching live listings.
- New AvitoListingGoneError (subclass of common AvitoError, NOT of
AvitoBlockedError/AvitoRateLimitedError) so the backfill block-trap
does not catch it.
- New _is_detail_not_found() detector + browser-mode branch in fetch_detail,
checked BEFORE firewall/soft-block (keyed on the 404 title so it does not
swallow the generic soft-block deflect). Curl path unchanged (404 → 302/
non-200 → ValueError → failed, as before).
- Backfill: new `gone` counter; dedicated except AvitoListingGoneError branch
that is neutral to the consecutive-block breaker and marks the listing
is_active=FALSE (SAVEPOINT) so it leaves the snapshot scope.
Tests: unit for _is_detail_not_found (404 True; soft-block/item-view/empty
False), browser-mode fetch_detail raising AvitoListingGoneError (not Blocked),
and backfill gone-row marks inactive without aborting the breaker.
