Route each scraper source (avito/cian/yandex/domclick) to its own camoufox
browser+proxy so they no longer wedge each other through a single global egress.
Feature-flagged (FEATURE_BROWSER_POOL_ENABLED, default OFF): with the flag off the
/fetch and /login paths are byte-for-byte the existing single-browser behavior.
When on, /fetch routes by body["source"] to a per-proxy browser via BROWSER_PROXY_MAP
(BROWSER_PROXY_AVITO/CIAN/YANDEX/DOMCLICK with legacy fallbacks), each guarded by its
own lazy-launched lock. /login stays single-browser in Phase 1.
BrowserFetcher gains a source arg (default avito) and sends it in the /fetch body;
all scraper callsites pass their source. No docker-compose/.env.runtime changes
(Phase 2, owner-gated).
