refactor: extract shared DB helpers to tasks/db.py

Extract identical get_async_session() and cleanup_engine() functions
from downloads.py and maintenance.py into a shared db.py module.
This eliminates code duplication and improves maintainability.

db.py imports only from app.config and SQLAlchemy to avoid circular
dependencies with the task modules that import from it.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/tasks/db.py

@@ -0,0 +1,27 @@
+"""Shared async database helpers for Celery tasks.
+
+Provides get_async_session() and cleanup_engine() used by both
+downloads.py and maintenance.py. This module must not import from
+either of those files to avoid circular imports.
+"""
+
+from sqlalchemy.ext.asyncio import create_async_engine, AsyncSession, async_sessionmaker
+
+from app.config import get_settings
+
+
+def get_async_session():
+    """Get async session factory for Celery tasks.
+
+    Creates a fresh engine and session factory each time to avoid
+    event loop issues when asyncio.run() creates new loops.
+    """
+    settings = get_settings()
+    engine = create_async_engine(settings.async_database_url, echo=False)
+    session_factory = async_sessionmaker(engine, class_=AsyncSession, expire_on_commit=False)
+    return session_factory, engine
+
+
+async def cleanup_engine(engine):
+    """Properly dispose of the async engine."""
+    await engine.dispose()