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/app/sync/playlists.py

@@ -20,7 +20,8 @@ from sqlalchemy.pool import NullPool
 
 from app import logs, progress, quota
 from app.auth import has_read_scope
-from app.models import User, Video
+from app.models import User
+from app.catalog.models import Video
 from app.reader.models import Playlist, PlaylistItem
 from app.sync.playlist_merge import merge_plan
 from app.sync.runner import token_users