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_search_scrape_ingest.py

@@ -14,7 +14,8 @@ a placeholder never becomes the video's permanent date.
 from datetime import datetime, timezone
 
 from app import sysconfig
-from app.models import User, Video
+from app.models import User
+from app.catalog.models import Video
 from app.routes.search import _ingest_candidates