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_failure_lines.py

@@ -20,7 +20,7 @@ import yt_dlp
 from app import logs, sysconfig, worker
 from app.downloads import pipeline
 from app.downloads.stated import StatedFailure
-from app.models import DownloadJob, MediaAsset
+from app.downloads.models import DownloadJob, MediaAsset
 from app.plex import stream as plex_stream