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_sync_claim.py

@@ -10,7 +10,7 @@ from sqlalchemy import text
 
 from app.auth import require_write_scope
 from app.db import SessionLocal
-from app.models import Playlist
+from app.reader.models import Playlist
 from app.routes import playlists as plroutes
 from app.sync import playlists as plsync