refactor(plex): the unit's tables move into plex/models

PlexLibrary, PlexShow, PlexSeason, PlexItem, PlexPerson, PlexCollection, PlexState, PlexLink,
PlexPlaylist and PlexPlaylistItem leave app/models.py for app/plex/models.py; app.tables registers
them. Every import is re-pointed; no schema change.

backend/app/discover/queries.py

@@ -29,7 +29,8 @@ from app.feedquery import (
     serialize,
     sort_keys,
 )
-from app.models import PlexItem, PlexShow, PlexState, User
+from app.models import User
+from app.plex.models import PlexItem, PlexShow, PlexState
 from app.catalog.models import Channel, Video
 from app.reader.models import PlaylistItem, VideoState
 from app.plex import rollup