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/routes/plex/admin.py

@@ -10,11 +10,8 @@ from sqlalchemy.orm import Session
 from app import logs, sysconfig
 from app.auth import admin_user, current_user
 from app.db import get_db
-from app.models import (
-    PlexItem,
-    PlexLink,
-    User,
-)
+from app.models import User
+from app.plex.models import PlexItem, PlexLink
 from app.plex import paths as plex_paths
 from app.plex import sync as plex_sync
 from app.plex import watch_sync as plex_watch