Avito listings re-seen by the scraper get is_active=true but were never
deactivated when they stop appearing (sold/removed) — unlike cian's snapshot
scope. Result: all avito rows stay is_active=true, inflating the coverage
denominator and polluting estimator analog medians with dead lots.
Add scheduled task deactivate_stale_avito_listings: marks source='avito'
is_active=false when last_seen_at older than AVITO_STALE_TTL_DAYS (10d),
window 06:00-07:00 UTC (after avito_city_sweep). No DELETE — history kept
for backtest #667. cian/yandex untouched. Wired into scheduler_loop dispatch
+ seed migration 090 (ON CONFLICT(source) idempotent).
Refs #759
