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_download_transitions.py

@@ -8,7 +8,8 @@ behaviour-equivalently. DB lane (`db` fixture) — skipped without Postgres.
 
 from sqlalchemy import text
 
-from app.models import DownloadJob, MediaAsset, User
+from app.models import User
+from app.downloads.models import DownloadJob, MediaAsset
 
 
 def _seed(db, *, job_status, asset_status="ready", ref_count=1, rel_path=None):