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/collections.py

@@ -14,14 +14,8 @@ from app.collation import collated, sorted_words
 from app import logs
 from app.auth import admin_user, current_user
 from app.db import get_db
-from app.models import (
-    PlexCollection,
-    PlexItem,
-    PlexLibrary,
-    PlexShow,
-    PlexState,
-    User,
-)
+from app.models import User
+from app.plex.models import PlexCollection, PlexItem, PlexLibrary, PlexShow, PlexState
 from app.plex import rollup
 from app.plex import sync as plex_sync
 from app.plex.cards import SHOW_NEW, display_title, movie_card, norm_lang, show_card