refactor(dry-B4): extract shared async_session_factory for Celery tasks

download/migration/scan each defined an identical _async_session_factory()
(fresh per-invocation async engine — async connections are event-loop-bound
so each asyncio.run() task needs its own engine, unlike the process-wide
_sync_engine). Moved it to tasks/_async_session.py; the 3 files import it
and drop their now-orphaned sqlalchemy.ext.asyncio / get_config imports
(migration keeps AsyncSession for a type hint). Call-site try/finally
dispose left as-is to avoid re-indenting the critical task bodies.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/tasks/download.py

@@ -5,10 +5,8 @@ from pathlib import Path
 
 from sqlalchemy import select
 from sqlalchemy.exc import DBAPIError, OperationalError
-from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
 
 from ..celery_app import celery
-from ..config import get_config
 from ..models import ImportSettings
 from ..services.credential_crypto import CredentialCrypto
 from ..services.credential_service import CredentialService
@@ -16,18 +14,13 @@ from ..services.download_service import DownloadService
 from ..services.gallery_dl import GalleryDLService
 from ..services.importer import Importer
 from ..services.thumbnailer import Thumbnailer
+from ._async_session import async_session_factory
 from .import_file import _sync_session_factory
 
 IMAGES_ROOT = Path("/images")
 _KEY_PATH = IMAGES_ROOT / "secrets" / "credential_key.b64"
 
 
-def _async_session_factory():
-    cfg = get_config()
-    engine = create_async_engine(cfg.database_url, future=True, pool_pre_ping=True)
-    return async_sessionmaker(engine, class_=AsyncSession, expire_on_commit=False), engine
-
-
 @celery.task(
     name="backend.app.tasks.download.download_source",
     bind=True,
@@ -44,7 +37,7 @@ def download_source(self, source_id: int) -> int:
     """Returns the DownloadEvent.id."""
 
     async def _run():
-        async_factory, async_engine = _async_session_factory()
+        async_factory, async_engine = async_session_factory()
         SyncFactory = _sync_session_factory()
         try:
             with SyncFactory() as sync_session: