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/plex/rollup.py

@@ -8,7 +8,7 @@ from datetime import datetime, timezone
 from sqlalchemy import and_, case, func, or_, select
 from sqlalchemy.orm import Session, aliased
 
-from app.models import PlexItem, PlexState
+from app.plex.models import PlexItem, PlexState
 from app.plex.cards import ShowState
 
 # Sort sentinel for a rail row whose timestamp is missing, so `key=lambda r: r[0] or NO_TS` reads the