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.

scripts/e2e_seed.py

@@ -24,13 +24,8 @@ sys.path.insert(0, "/app")
 from sqlalchemy import delete, func, select
 
 from app.db import SessionLocal
-from app.models import (
-    AppState,
-    Channel,
-    Subscription,
-    User,
-    Video,
-)
+from app.catalog.models import Channel, Subscription, Video
+from app.models import AppState, User
 from app.reader.models import Playlist, PlaylistItem, VideoState
 from app.security import hash_password