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_relevance_window.py

@@ -12,7 +12,8 @@ from datetime import datetime, timezone
 import pytest
 
 from app import feedquery
-from app.models import Channel, Subscription, User, Video
+from app.models import User
+from app.catalog.models import Channel, Subscription, Video
 
 
 def _video(vid: str, day: int, title: str) -> Video: