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_feed_keyset.py

@@ -16,7 +16,8 @@ from datetime import datetime, timezone
 
 import pytest
 
-from app.models import Channel, Subscription, User, Video
+from app.models import User
+from app.catalog.models import Channel, Subscription, Video
 
 # Every sort feedquery.sort_keys defines except "relevance" (which needs a search query to rank).
 _SORTS = [