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/cards.py

@@ -22,7 +22,8 @@ to the client, which has both the native payload and the translations.
 """
 
 from app.discover.budget import remaining_seconds
-from app.models import Channel, Playlist
+from app.models import Channel
+from app.reader.models import Playlist
 
 # Plex artwork is a poster for the things you browse and a landscape still for the things you play.
 _POSTER_KINDS = ("movie", "show")

backend/app/discover/pins.py

@@ -22,7 +22,8 @@ from sqlalchemy import func, select
 from sqlalchemy.orm import Session
 
 from app.discover import cards, queries
-from app.models import Channel, ComfortPin, PlexItem, PlexShow, Playlist, User, Video
+from app.models import Channel, PlexItem, PlexShow, User, Video
+from app.reader.models import ComfortPin, Playlist
 from app.plex import rollup
 from app.plex.cards import SHOW_NEW, leaf_card, show_card
 from app.plex.lookups import by_rating_key

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