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_worker_dispatch.py

@@ -12,7 +12,7 @@ from app.config import settings
 from app.downloads import pipeline, runner, service, storage
 from app.downloads import probe as probemod
 from app.downloads import upload as uploadmod
-from app.models import DownloadJob, MediaAsset
+from app.downloads.models import DownloadJob, MediaAsset
 
 
 @pytest.mark.parametrize(