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/app/sync/explore.py

@@ -14,15 +14,8 @@ from sqlalchemy import delete, or_, select
 from sqlalchemy.orm import Session
 
 from app import logs, sysconfig
-from app.models import (
-    Channel,
-    ExploredChannel,
-    PlaylistItem,
-    SearchFind,
-    Subscription,
-    Video,
-    VideoState,
-)
+from app.models import Channel, ExploredChannel, SearchFind, Subscription, Video
+from app.reader.models import PlaylistItem, VideoState
 from app.sync.videos import insert_stubs, stub_from_playlist_item, apply_video_details
 from app.youtube.client import YouTubeClient, YouTubeError, is_playlist_gone