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_edit_cap.py

@@ -13,7 +13,7 @@ from app.config import settings
 from app.downloads import errors as dlerrors
 from app.downloads import pipeline, quota, service
 from app.downloads import upload as uploadmod
-from app.models import DownloadJob, DownloadQuota, MediaAsset
+from app.downloads.models import DownloadJob, DownloadQuota, MediaAsset
 
 pytestmark = pytest.mark.skipif(shutil.which("ffmpeg") is None, reason="ffmpeg not available")