refactor(tradein/avito): avito_detail_backfill off scrape_pipeline helpers (#2397 slice B)

app/tasks/avito_detail_backfill.py imported _CHROME_HEADERS/_avito_proxies from
the legacy app/services/scrape_pipeline.py (the only two symbols it needed from
that module). scrape_pipeline.py is slated for wholesale deletion in Part E of
epic #2277 -- this severs the last dependency so that deletion won't break the
backfill task.

Source chosen: kit-reuse, not relocate. scraper_kit/providers/_base.py (#2358
Foundation) already extracted the identical building blocks while deduplicating
providers/avito/{serp,detail,imv}.py:
- DOCUMENT_HEADERS -- byte-identical dict to _CHROME_HEADERS (same 8 keys/values,
  Accept/Accept-Language/Cache-Control/Sec-Fetch-*/Upgrade-Insecure-Requests).
- http_proxies(proxy_url) -- same formula as _avito_proxies():
  {"http": url, "https": url} if url else None, just parameterized instead of
  reading settings.scraper_proxy_url internally (both resolve the identical
  settings singleton in production, so behavior is unchanged).

Kept the AsyncSession(...) construction inline (didn't switch to kit's
build_document_session helper) to preserve the existing
`app.tasks.avito_detail_backfill.AsyncSession` mock seam used by
tests/tasks/test_avito_detail_backfill.py -- swapping to the helper would call
curl_cffi's AsyncSession via scraper_kit.providers._base instead, silently
bypassing that patch target.

scrape_pipeline.py itself is untouched (kept its own copies, per Part E plan).

Full backend suite: 3276 passed, 1 known-unrelated fail (test_search_cache_hit,
#2208), 6 skipped.

tradein-mvp/backend/app/tasks/avito_detail_backfill.py

@@ -29,6 +29,14 @@ from scraper_kit.avito_exceptions import (
     AvitoRateLimitedError,
 )
 from scraper_kit.browser_fetcher import BrowserFetcher
+
+# #2397 slice B (эпик #2277 decommission scrape_pipeline.py, Part E): раньше
+# _CHROME_HEADERS/_avito_proxies() импортировались из app.services.scrape_pipeline.
+# kit уже вынес byte-идентичный dict (DOCUMENT_HEADERS) и ту же формулу
+# {"http": url, "https": url} if url else None (http_proxies) в
+# providers/_base.py (#2358 Foundation) — реюзаем вместо копии, разрывая
+# зависимость от scrape_pipeline.py перед его удалением.
+from scraper_kit.providers._base import DOCUMENT_HEADERS, http_proxies
 from scraper_kit.providers.avito.detail import (
     fetch_detail,
     research_in_session,
@@ -41,7 +49,6 @@ from sqlalchemy.orm import Session
 from app.core.config import settings
 from app.core.shutdown import shutdown_requested
 from app.services import scrape_runs as runs_mod
-from app.services.scrape_pipeline import _CHROME_HEADERS, _avito_proxies
 from app.services.scraper_adapters import RealScraperConfig
 
 # _AVITO_WARM_SEARCH_URL/build_warmed_session остаются legacy (#2310): kit's
@@ -174,8 +181,8 @@ async def run_avito_detail_backfill(
             session = AsyncSession(
                 impersonate="chrome120",
                 timeout=25,
-                headers=_CHROME_HEADERS,
-                proxies=_avito_proxies(),
+                headers=DOCUMENT_HEADERS,
+                proxies=http_proxies(settings.scraper_proxy_url),
             )
             scraper._cffi = session