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/playback.py

@@ -22,13 +22,8 @@ from sqlalchemy.orm import Session
 
 from app.auth import current_user, resolved_user_id
 from app.db import SessionLocal, get_db
-from app.models import (
-    PlexItem,
-    PlexSeason,
-    PlexShow,
-    PlexState,
-    User,
-)
+from app.models import User
+from app.plex.models import PlexItem, PlexSeason, PlexShow, PlexState
 from app.plex import imgcache, subtitles
 from app.plex import paths as plex_paths
 from app.plex.lookups import by_rating_key