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_unit_fences.py

@@ -17,7 +17,8 @@ still not reach into a unit's tables.
 import ast
 from pathlib import Path
 
-from app.models import Base
+import app.tables  # noqa: F401 — every unit's tables, for the ownership check
+from app.db import Base
 
 APP = Path(__file__).parent.parent / "app"
 ROUTES = APP / "routes"