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/playlists.py

@@ -12,7 +12,8 @@ from app import quota
 from app.auth import current_user, has_read_scope, has_write_scope, require_write_scope
 from app.db import get_db
 from app.feedquery import feed_columns, serialize
-from app.models import Channel, Playlist, PlaylistItem, User, Video, VideoState
+from app.models import Channel, User, Video
+from app.reader.models import Playlist, PlaylistItem, VideoState
 from app.sync.playlists import (
     PlaylistGone,
     lock_playlists,