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_plex_people_enrich.py

@@ -9,7 +9,8 @@ from datetime import datetime, timezone
 import httpx
 import pytest
 
-from app.models import PlexItem, PlexLibrary, PlexPerson, User
+from app.models import User
+from app.plex.models import PlexItem, PlexLibrary, PlexPerson
 from app.plex import imgcache
 from app.plex import people as plex_people
 from app.plex import tmdb as plex_tmdb