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_empty_listing_guards.py

@@ -12,15 +12,8 @@ contradicts the remote's own count is not believed either.
 import pytest
 from sqlalchemy import select
 
-from app.models import (
-    Channel,
-    Playlist,
-    PlaylistItem,
-    PlexCollection,
-    PlexItem,
-    PlexLibrary,
-    Video,
-)
+from app.models import Channel, PlexCollection, PlexItem, PlexLibrary, Video
+from app.reader.models import Playlist, PlaylistItem
 from app.plex import sync as plex_sync
 from app.sync import playlists as plsync
 from app.youtube.client import YouTubeError