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/app/tables.py

@@ -0,0 +1,9 @@
+"""Every table, registered on `Base.metadata`.
+
+Each unit declares its tables in its own `models` module, so importing `app.models` alone no longer
+names the whole schema. Whatever needs all of it — the migrations, the test database, a check over
+every mapper — imports this module.
+"""
+
+import app.downloads.models  # Capture's tables
+import app.models  # noqa: F401 — the core tables