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_phone_safe_contract.py

@@ -17,7 +17,7 @@ import pytest
 
 from app.config import settings
 from app.downloads import pipeline, playable, probe
-from app.models import DownloadJob, MediaAsset
+from app.downloads.models import DownloadJob, MediaAsset
 
 APP = Path(__file__).resolve().parents[1] / "app"