refactor(reader): the unit's tables move into reader/models

VideoState, Playlist, PlaylistItem, SavedView, ComfortPin, Tag and ChannelTag leave app/models.py
for app/reader/models.py; app.tables registers them. No schema change.

backend/app/routes/discover.py

@@ -22,7 +22,8 @@ from app.db import get_db
 from app.discover import hero as hero_mod
 from app.discover import pins, rails
 from app.discover.budget import Budget, DEFAULT_BUDGET
-from app.models import ComfortPin, User
+from app.models import User
+from app.reader.models import ComfortPin
 from app.plex.cards import norm_lang
 from app.routes._common import owned_or_404, not_found