fix(tasks): share one sync engine per worker process to stop connection-pool leak

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/tasks/maintenance.py

@@ -6,13 +6,12 @@ from datetime import UTC, datetime, timedelta
 from pathlib import Path
 
 from PIL import Image
-from sqlalchemy import create_engine, delete, select, update
-from sqlalchemy.orm import sessionmaker
+from sqlalchemy import delete, select, update
 
 from ..celery_app import celery
-from ..config import get_config
 from ..models import DownloadEvent, ImageRecord, ImportSettings, ImportTask
 from ..utils.phash import compute_phash
+from ._sync_engine import sync_session_factory as _sync_session_factory
 
 log = logging.getLogger(__name__)
 
@@ -23,12 +22,6 @@ VERIFY_PAGE = 200
 FFPROBE_TIMEOUT_SECONDS = 10
 
 
-def _sync_session_factory():
-    cfg = get_config()
-    engine = create_engine(cfg.database_url_sync, future=True, pool_pre_ping=True)
-    return sessionmaker(engine, expire_on_commit=False)
-
-
 @celery.task(name="backend.app.tasks.maintenance.recover_interrupted_tasks")
 def recover_interrupted_tasks() -> int:
     """Find ImportTask rows stuck in 'processing' for >30 min and re-queue them.