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/discover/queries.py

@@ -29,16 +29,8 @@ from app.feedquery import (
     serialize,
     sort_keys,
 )
-from app.models import (
-    Channel,
-    PlaylistItem,
-    PlexItem,
-    PlexShow,
-    PlexState,
-    User,
-    Video,
-    VideoState,
-)
+from app.models import Channel, PlexItem, PlexShow, PlexState, User, Video
+from app.reader.models import PlaylistItem, VideoState
 from app.plex import rollup
 from app.plex.cards import display_title, leaf_card, movie_card, show_card
 from app.plex.query import apply_show_state_filter, content_libs