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_playlist_add_position.py

@@ -8,7 +8,7 @@ the position is an index over the ORDERED rows, and the list is renumbered aroun
 
 from datetime import datetime, timezone
 
-from app.models import Channel, Video
+from app.catalog.models import Channel, Video
 from app.reader.models import Playlist, PlaylistItem