refactor(catalog): the unit's tables move into catalog/models

Channel, Subscription, Video, SearchFind, ExploredChannel and BlockedChannel (and LIVE_OR_UPCOMING)
leave app/models.py for app/catalog/models.py; app.tables registers them. The User↔Subscription
relationship names its other side as a type only. No schema change.

backend/tests/test_channel_subscribe_upstream.py

@@ -9,7 +9,7 @@ import pytest
 from fastapi import HTTPException
 from sqlalchemy import select
 
-from app.models import Channel, Subscription
+from app.catalog.models import Channel, Subscription
 from app.routes import channels
 from app.youtube.client import YouTubeError