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/app/sync/runner.py

@@ -16,7 +16,8 @@ from sqlalchemy.orm import Session
 
 from app import logs, progress, quota, sysconfig
 from app.db import SessionLocal
-from app.models import Channel, OAuthToken, Subscription, User
+from app.models import OAuthToken, User
+from app.catalog.models import Channel, Subscription
 from app.sync.explore import followed_channel
 from app.sync.subscriptions import import_subscriptions
 from app.sync.videos import (