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_upload_conversion.py

@@ -21,7 +21,8 @@ from app.downloads import pipeline, quota, storage
 from app.downloads import probe as probemod
 from app.downloads import upload as uploadmod
 from app.downloads.stated import StatedFailure
-from app.models import DownloadJob, DownloadQuota, MediaAsset, Notification
+from app.models import Notification
+from app.downloads.models import DownloadJob, DownloadQuota, MediaAsset
 from sqlalchemy import select, update
 from sqlalchemy.exc import OperationalError
 from sqlalchemy.orm import Session