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/subscriptions.py

@@ -7,7 +7,8 @@ from sqlalchemy import or_, select
 from sqlalchemy.orm import Session
 
 from app import logs
-from app.models import Channel, Subscription, User
+from app.models import User
+from app.catalog.models import Channel, Subscription
 from app.utils import normalize_description
 from app.youtube.client import YouTubeClient, YouTubeError, best_thumbnail