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/worker.py

@@ -46,7 +46,8 @@ from app.downloads.jobdb import (
     _staging_dir,
 )
 from app.downloads.status import STOPPED_BY_ROUTE, JobKind, JobStatus
-from app.models import DISPLAY_TEXT_MAX, DownloadJob, MediaAsset
+from app.models import DISPLAY_TEXT_MAX
+from app.downloads.models import DownloadJob, MediaAsset
 
 log = logs.get_logger(__name__)
 _stop = threading.Event()