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_playlist_move.py

@@ -16,7 +16,8 @@ from datetime import datetime, timedelta, timezone
 import pytest
 from sqlalchemy.exc import OperationalError
 
-from app.models import Channel, Playlist, PlaylistItem, Video
+from app.models import Channel, Video
+from app.reader.models import Playlist, PlaylistItem
 
 
 def _seed_videos(db, ids):