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/subtitles.py

@@ -47,7 +47,7 @@ from sqlalchemy.orm import Session
 
 from app import logs
 from app.db import SessionLocal
-from app.models import PlexItem
+from app.plex.models import PlexItem
 from app.plex import paths as plex_paths
 from app.plex.cache import CACHE_DIR
 from app.plex.client import PlexClient, PlexError