refactor(capture): the unit's tables move into downloads/models

The nine Capture tables (files, jobs, folders, shares, links, quota, cookies, profiles, library
resume points) leave app/models.py for the unit's own module; app.tables registers every unit's
tables for the migrations, the test database and the ownership check. No schema change.

backend/tests/test_download_job_locks.py

@@ -18,7 +18,8 @@ from sqlalchemy import select
 from app.apitokens import FilesCaller
 from app.db import SessionLocal
 from app.downloads import service
-from app.models import DownloadJob, MediaAsset, User
+from app.models import User
+from app.downloads.models import DownloadJob, MediaAsset
 from app.routes.downloads import cancel_download, pause_download
 from app.routes.files import delete_file, own_job_for_update