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/scan.py

@@ -11,14 +11,14 @@ import asyncio
 from datetime import UTC, datetime
 from pathlib import Path
 
-from sqlalchemy import create_engine, select
+from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
-from sqlalchemy.orm import sessionmaker
 
 from ..celery_app import celery
 from ..config import get_config
 from ..models import DownloadEvent, ImportBatch, ImportSettings, ImportTask
 from ..services.scheduler_service import select_due_sources
+from ._sync_engine import sync_session_factory as _sync_session_factory
 
 
 def _iter_import_files(import_root: Path):
@@ -35,12 +35,6 @@ def _iter_import_files(import_root: Path):
         yield entry
 
 
-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.scan.scan_directory", bind=True)
 def scan_directory(self, triggered_by: str = "manual",
                    mode: str = "quick") -> int: