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/migration.py

@@ -15,14 +15,14 @@ from datetime import UTC, datetime
 from pathlib import Path
 
 from sqlalchemy import select
-from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
+from sqlalchemy.ext.asyncio import AsyncSession
 
 from ..celery_app import celery
-from ..config import get_config
 from ..models import MigrationRun
 from ..services.credential_crypto import CredentialCrypto
 from ..services.migrators import cleanup as cleanup_mod
 from ..services.migrators import gs_ingest, ir_ingest, ml_queue, tag_apply, verify
+from ._async_session import async_session_factory
 
 log = logging.getLogger(__name__)
 
@@ -30,12 +30,6 @@ 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
-
-
 async def _update_run(
     db: AsyncSession, run_id: int, *,
     status: str | None = None, counts: dict | None = None,
@@ -59,7 +53,7 @@ async def _update_run(
 
 
 async def _run_async(run_id: int, kind: str, params: dict) -> dict:
-    factory, engine = _async_session_factory()
+    factory, engine = async_session_factory()
     try:
         async with factory() as db:
             await _update_run(db, run_id, status="running")