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/watch_sync.py

@@ -29,7 +29,7 @@ from sqlalchemy.orm import Session
 
 from app import logs, sysconfig
 from app.db import SessionLocal
-from app.models import PlexItem, PlexLibrary, PlexLink, PlexState
+from app.plex.models import PlexItem, PlexLibrary, PlexLink, PlexState
 from app.plex.client import PlexClient, PlexError, PlexNotConfigured
 from app.plex.sync import (
     _TYPE_EPISODE,