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_finish.py

@@ -13,7 +13,7 @@ from datetime import datetime, timedelta, timezone
 from sqlalchemy import select
 
 from app.db import SessionLocal
-from app.models import PlexItem, PlexLibrary, PlexState
+from app.plex.models import PlexItem, PlexLibrary, PlexState
 from app.routes.plex import playback