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/plex/overlay.py

@@ -13,7 +13,7 @@ from sqlalchemy import select
 from sqlalchemy.orm import Session
 
 from app import logs, sysconfig
-from app.models import PlexItem, PlexLibrary, PlexShow
+from app.plex.models import PlexItem, PlexLibrary, PlexShow
 from app.plex import sync as plex_sync
 from app.plex.client import PlexClient
 from app.plex.genres import hu_genres