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/app/discover/queries.py

@@ -29,7 +29,8 @@ from app.feedquery import (
     serialize,
     sort_keys,
 )
-from app.models import Channel, PlexItem, PlexShow, PlexState, User, Video
+from app.models import PlexItem, PlexShow, PlexState, User
+from app.catalog.models import Channel, Video
 from app.reader.models import PlaylistItem, VideoState
 from app.plex import rollup
 from app.plex.cards import display_title, leaf_card, movie_card, show_card