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_failure_catalog.py

@@ -24,7 +24,8 @@ from app import security
 from app.config import settings
 from app.db import engine
 from app.downloads import credentials, formats, pipeline, service
-from app.models import DownloadJob, DownloadQuota, MediaAsset, User
+from app.models import User
+from app.downloads.models import DownloadJob, DownloadQuota, MediaAsset
 from app.worker import _process_job
 
 _REEL = "https://www.facebook.com/reel/1736360027626346/?fs=e&fs=e"