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/routes/feed.py

@@ -25,13 +25,8 @@ from app.feedquery import (
     windowed_rank,
     windowed_relevance_rows,
 )
-from app.models import (
-    ChannelTag,
-    Tag,
-    User,
-    Video,
-    VideoState,
-)
+from app.models import User, Video
+from app.reader.models import ChannelTag, Tag, VideoState
 from app.routes._common import (
     ProgressIn,
     get_or_404,