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/app/routes/plex/playlists.py

@@ -11,14 +11,8 @@ from sqlalchemy.orm import Session
 
 from app.auth import current_user
 from app.db import get_db
-from app.models import (
-    PlexItem,
-    PlexPlaylist,
-    PlexPlaylistItem,
-    PlexShow,
-    PlexState,
-    User,
-)
+from app.models import User
+from app.plex.models import PlexItem, PlexPlaylist, PlexPlaylistItem, PlexShow, PlexState
 from app.plex.cards import display_title, leaf_card, norm_lang
 from app.plex.lookups import by_rating_key
 from app.routes._common import owned_or_404, not_found