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/runner.py

@@ -23,7 +23,7 @@ from app.db import SessionLocal
 from app.downloads import edit as editmod
 from app.downloads import formats
 from app.downloads.status import JobKind
-from app.models import DownloadJob
+from app.downloads.models import DownloadJob
 
 log = logs.get_logger(__name__)