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>
