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_merge.py

@@ -10,7 +10,8 @@ from datetime import datetime, timezone
 
 import pytest
 
-from app.models import Channel, Playlist, PlaylistItem, Video
+from app.models import Channel, Video
+from app.reader.models import Playlist, PlaylistItem
 from app.sync import playlists as pl_mod
 from app.sync.playlist_merge import merge_plan
 from app.sync.playlists import (