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_file_folders.py

@@ -17,7 +17,8 @@ from app.downloads import folders, formats, service
 from app.downloads import gc as gcmod
 from app.db import SessionLocal
 from app.downloads.status import JobStatus
-from app.models import DownloadJob, FileFolder, MediaAsset, Notification
+from app.models import Notification
+from app.downloads.models import DownloadJob, FileFolder, MediaAsset
 
 NOW = datetime.now(timezone.utc)