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_tmdb.py

@@ -9,7 +9,7 @@ from datetime import datetime, timezone
 import httpx
 import pytest
 
-from app.models import PlexItem, PlexLibrary, PlexPerson, PlexShow
+from app.plex.models import PlexItem, PlexLibrary, PlexPerson, PlexShow
 from app.plex import tmdb
 
 # A TMDB /movie/{id}?append_to_response=keywords,credits,recommendations,images,external_ids payload.