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_channel_backfill.py

@@ -12,7 +12,8 @@ import app.state as state
 from app.auth import current_user
 from app.db import get_db
 from app.main import app
-from app.models import Channel, Subscription, User, Video
+from app.models import User
+from app.catalog.models import Channel, Subscription, Video
 from app.sync import runner, videos
 
 _PUB = datetime(2020, 1, 1, tzinfo=timezone.utc)