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/maintenance.py

@@ -7,9 +7,8 @@ from datetime import datetime, timedelta
 from pathlib import Path
 
 from sqlalchemy import select, update
-from sqlalchemy.ext.asyncio import create_async_engine, AsyncSession, async_sessionmaker
 
-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.setting import Setting, STORAGE_STATS_SETTING
 from app.models.download import Download, DownloadStatus
@@ -17,6 +16,7 @@ from app.models.base import utcnow
 
 logger = logging.getLogger(__name__)
 
+from app.config import get_settings
 settings = get_settings()
 
 # Maximum time a job can be in "running" state before considered orphaned
@@ -29,18 +29,6 @@ STALE_RUNNING_THRESHOLD_MINUTES = 90
 STALE_QUEUED_THRESHOLD_MINUTES = 30
 
 
-def get_async_session():
-    """Get async session factory for Celery tasks."""
-    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()
-
-
 def format_size(size_bytes: int) -> str:
     """Format bytes as human-readable size."""
     for unit in ['B', 'KB', 'MB', 'GB', 'TB']: