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/maintenance.py

@@ -27,7 +27,7 @@ from sqlalchemy.orm import Session
 
 from app import logs, progress, quota, sysconfig
 from app.config import settings
-from app.models import Video
+from app.catalog.models import Video
 from app.reader import queries as reader_queries
 from app.notifications import create_notification
 from app.sync.runner import get_service_user