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/_async_session.py

@@ -0,0 +1,21 @@
+"""Per-invocation async session factory for Celery task modules.
+
+Async engine connections are bound to the event loop. Each Celery task
+runs its async body under a fresh ``asyncio.run()`` loop, so it needs its
+own engine created (and disposed) within that loop — a process-wide async
+engine would reuse loop-bound connections across tasks and raise "attached
+to a different loop". So unlike the process-wide sync engine in
+``_sync_engine.py``, this returns a fresh engine per call; the caller
+disposes it (``await engine.dispose()``) when its loop ends.
+"""
+
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
+
+from ..config import get_config
+
+
+def async_session_factory():
+    """Return ``(sessionmaker, engine)`` bound to a fresh async engine."""
+    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