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/import_file.py

@@ -5,20 +5,13 @@ updates the ImportTask state machine + ImportBatch counters atomically.
 from datetime import UTC, datetime
 from pathlib import Path
 
-from sqlalchemy import create_engine, select, update
-from sqlalchemy.orm import sessionmaker
+from sqlalchemy import select, update
 
 from ..celery_app import celery
-from ..config import get_config
 from ..models import ImportBatch, ImportSettings, ImportTask
 from ..services.importer import Importer
 from ..services.thumbnailer import Thumbnailer
-
-
-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)
+from ._sync_engine import sync_session_factory as _sync_session_factory
 
 
 IMAGES_ROOT = Path("/images")