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_collation.py

@@ -9,17 +9,8 @@ from app.auth import current_user
 from app.collation import sorted_words
 from app.db import get_db
 from app.main import app
-from app.models import (
-    Channel,
-    Playlist,
-    PlaylistItem,
-    PlexCollection,
-    PlexItem,
-    PlexLibrary,
-    Subscription,
-    User,
-    Video,
-)
+from app.models import Channel, PlexCollection, PlexItem, PlexLibrary, Subscription, User, Video
+from app.reader.models import Playlist, PlaylistItem
 from datetime import datetime, timezone
 
 WORDS = ["Zorró", "Életrajzi", "Ötödik", "oroszlán", "Csillag", "Cukor"]