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/alembic/env.py

@@ -4,7 +4,7 @@ from sqlalchemy import engine_from_config, pool
 from app.config import settings
 from app.db import Base
 from app.logs import configure_logging
-import app.models  # noqa: F401  (import so models register on Base.metadata)
+import app.tables  # noqa: F401  (import so every table registers on Base.metadata)
 
 config = context.config
 config.set_main_option("sqlalchemy.url", settings.database_url)