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_merge.py

@@ -10,7 +10,7 @@ from datetime import datetime, timezone
 
 import pytest
 
-from app.models import Channel, Video
+from app.catalog.models import Channel, Video
 from app.reader.models import Playlist, PlaylistItem
 from app.sync import playlists as pl_mod
 from app.sync.playlist_merge import merge_plan