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/app/downloads/credentials.py

@@ -33,7 +33,7 @@ from app import logs
 from app.downloads import clip as clipmod
 from app.downloads import formats
 from app.downloads.adapters import _host_is
-from app.models import DownloadCredential, MediaAsset
+from app.downloads.models import DownloadCredential, MediaAsset
 from app.security import decrypt_optional, encrypt
 
 log = logs.get_logger(__name__)