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_position_lock.py

@@ -19,16 +19,8 @@ from datetime import datetime, timezone
 import pytest
 from sqlalchemy import event
 
-from app.models import (
-    Channel,
-    Playlist,
-    PlaylistItem,
-    PlexItem,
-    PlexLibrary,
-    PlexPlaylist,
-    PlexPlaylistItem,
-    Video,
-)
+from app.models import Channel, PlexItem, PlexLibrary, PlexPlaylist, PlexPlaylistItem, Video
+from app.reader.models import Playlist, PlaylistItem
 
 
 @pytest.fixture