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_youtube_refusals.py

@@ -8,7 +8,8 @@ import httpx
 import pytest
 
 from app.auth import has_read_scope
-from app.models import Channel, OAuthToken
+from app.models import OAuthToken
+from app.catalog.models import Channel
 from app.sync.explore import explore_ingest_page
 from app.sync.videos import backfill_channel_deep, backfill_channel_recent
 from app.youtube import client as yt