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/routes/channels.py

@@ -15,13 +15,13 @@ from app.mailtext import lang_for_user
 from app import logs, quota, scheduler
 from app.auth import admin_user, current_user, require_write_scope
 from app.db import get_db
-from app.models import (
+from app.models import User
+from app.catalog.models import (
     LIVE_OR_UPCOMING,
     BlockedChannel,
     Channel,
     ExploredChannel,
     Subscription,
-    User,
     Video,
 )
 from app.reader.models import ChannelTag, Playlist, PlaylistItem, Tag