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_watch_sync.py

@@ -13,7 +13,8 @@ from types import SimpleNamespace
 
 from sqlalchemy import select
 
-from app.models import PlexItem, PlexLibrary, PlexState, User
+from app.models import User
+from app.plex.models import PlexItem, PlexLibrary, PlexState
 from app.plex.watch_sync import _pull_apply, _reconcile_inprogress_movies
 from app.routes.plex.playback import (
     _PUSH_THROTTLE_S,