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_sort_indexes.py

@@ -17,7 +17,7 @@ from sqlalchemy.dialects import postgresql
 from app import collation
 from app.db import engine
 from app.feedquery import order_by, sort_keys
-from app.models import Video
+from app.catalog.models import Video
 
 VIDEO_SORTS = [
     "newest",