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/thumbnail.py

@@ -7,24 +7,15 @@ so they deserve their own queue lane.
 
 from pathlib import Path
 
-from sqlalchemy import create_engine
-from sqlalchemy.orm import sessionmaker
-
 from ..celery_app import celery
-from ..config import get_config
 from ..models import ImageRecord
 from ..services.importer import is_video
 from ..services.thumbnailer import Thumbnailer
+from ._sync_engine import sync_session_factory as _sync_session_factory
 
 IMAGES_ROOT = Path("/images")
 
 
-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.thumbnail.generate_thumbnail", bind=True)
 def generate_thumbnail(self, image_id: int) -> dict:
     SessionLocal = _sync_session_factory()