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/downloads.py

@@ -6,10 +6,9 @@ from datetime import datetime, timedelta
 from typing import Optional
 
 from sqlalchemy import select, and_, or_, update
-from sqlalchemy.ext.asyncio import create_async_engine, AsyncSession, async_sessionmaker
 from sqlalchemy.orm import selectinload
 
-from app.config import get_settings
+from app.tasks.db import get_async_session, cleanup_engine as _cleanup_engine
 from app.tasks.celery_app import celery_app
 from app.models.base import utcnow
 from app.models.source import Source
@@ -23,25 +22,10 @@ from app.events import publish_event
 logger = logging.getLogger(__name__)
 
 # Settings for database connection
+from app.config import get_settings
 settings = 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.
-    """
-    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()
-
-
 async def _get_db_setting(session: AsyncSession, key: str, default=None):
     """Get a setting value from the database, falling back to defaults.