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_feed_views.py

@@ -14,7 +14,8 @@ import pytest
 
 from app import feedquery
 
-from app.models import Channel, ChannelTag, Subscription, Tag, Video, VideoState
+from app.models import Channel, Subscription, Video
+from app.reader.models import ChannelTag, Tag, VideoState
 
 
 def _dt(day: int) -> datetime: