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/tests/test_maintenance_lifecycle.py

@@ -12,7 +12,8 @@ from datetime import datetime, timedelta, timezone
 import pytest
 from sqlalchemy import select
 
-from app.models import Channel, Notification, Playlist, PlaylistItem, Video
+from app.models import Channel, Notification, Video
+from app.reader.models import Playlist, PlaylistItem
 from app.sync import maintenance