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_empty_listing_guards.py

@@ -12,7 +12,8 @@ contradicts the remote's own count is not believed either.
 import pytest
 from sqlalchemy import select
 
-from app.models import Channel, PlexCollection, PlexItem, PlexLibrary, Video
+from app.models import PlexCollection, PlexItem, PlexLibrary
+from app.catalog.models import Channel, Video
 from app.reader.models import Playlist, PlaylistItem
 from app.plex import sync as plex_sync
 from app.sync import playlists as plsync