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_credentials.py

@@ -18,7 +18,8 @@ from cryptography.fernet import Fernet
 
 from app import security
 from app.downloads import credentials, formats, service
-from app.models import DownloadCredential, DownloadJob, MediaAsset, User
+from app.models import User
+from app.downloads.models import DownloadCredential, DownloadJob, MediaAsset
 
 
 @pytest.fixture(autouse=True)