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/ml.py

@@ -8,23 +8,16 @@ apply_allowlist_tags sweeps which are 'maintenance' lane. Sync sessions
 
 from pathlib import Path
 
-from sqlalchemy import create_engine, select
-from sqlalchemy.orm import sessionmaker
+from sqlalchemy import select
 
 from ..celery_app import celery
-from ..config import get_config
 from ..models import ImageRecord, MLSettings
+from ._sync_engine import sync_session_factory as _sync_session_factory
 
 IMAGES_ROOT = Path("/images")
 VIDEO_EXTS = {".mp4", ".mov", ".avi", ".mkv", ".webm", ".m4v", ".wmv", ".flv"}
 
 
-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)
-
-
 def _is_video(path: Path) -> bool:
     return path.suffix.lower() in VIDEO_EXTS