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/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 PlexItem, PlexShow, User
+from app.models import User
+from app.plex.models import PlexItem, PlexShow
 from app.catalog.models import Channel, Video
 from app.reader.models import ComfortPin, Playlist
 from app.plex import rollup