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/saved_views.py

@@ -6,7 +6,8 @@ from sqlalchemy.orm import Session
 
 from app.auth import current_user
 from app.db import get_db
-from app.models import SavedView, User
+from app.models import User
+from app.reader.models import SavedView
 from app.routes._common import owned_or_404
 from app.refusal import refused