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_list_index_only.py

@@ -14,7 +14,7 @@ from sqlalchemy import text
 from sqlalchemy.dialects import postgresql
 
 from app.feedquery import filtered_query
-from app.models import Video
+from app.catalog.models import Video
 from app.routes.channels import channel_aggregates
 
 INDEX = "ix_videos_channel_pub_cover"