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/tests/test_playlist_position_lock.py

@@ -19,7 +19,7 @@ from datetime import datetime, timezone
 import pytest
 from sqlalchemy import event
 
-from app.models import PlexItem, PlexLibrary, PlexPlaylist, PlexPlaylistItem
+from app.plex.models import PlexItem, PlexLibrary, PlexPlaylist, PlexPlaylistItem
 from app.catalog.models import Channel, Video
 from app.reader.models import Playlist, PlaylistItem