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")