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/reader/queries.py

@@ -11,7 +11,8 @@ from sqlalchemy import ColumnElement, and_, func, or_, select
 from sqlalchemy.orm import InstrumentedAttribute, Session, aliased
 
 from app.feedquery import feed_columns
-from app.models import Channel, User, Video
+from app.models import User
+from app.catalog.models import Channel, Video
 from app.reader.models import ChannelTag, Playlist, PlaylistItem, SavedView, Tag, VideoState
 from app.userscope import owned_by, owned_row