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/scan.py

@@ -12,13 +12,12 @@ 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 ..celery_app import celery
-from ..config import get_config
 from ..models import DownloadEvent, ImportBatch, ImportSettings, ImportTask
 from ..services.archive_extractor import is_archive
 from ..services.scheduler_service import record_tick, select_due_sources
+from ._async_session import async_session_factory
 from ._sync_engine import sync_session_factory as _sync_session_factory
 
 
@@ -141,14 +140,8 @@ def scan_directory(self, triggered_by: str = "manual",
 # --- FC-3d: periodic source-check tick ------------------------------------
 
 
-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 _tick_due_sources_async() -> dict:
-    factory, engine = _async_session_factory()
+    factory, engine = async_session_factory()
     try:
         async with factory() as session:
             # Prove the scheduler is alive even on empty ticks (UI reads this).