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_library_progress.py

@@ -9,7 +9,7 @@ cannot reach, is ever read or written. DB-backed (real Postgres).
 import pytest
 from sqlalchemy import select
 
-from app.models import DownloadJob, DownloadShare, LibraryState, MediaAsset
+from app.downloads.models import DownloadJob, DownloadShare, LibraryState, MediaAsset
 from app.routes._common import ProgressIn, kept_position