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/tests/test_plex_admin_routes.py

@@ -9,7 +9,7 @@ from sqlalchemy import select
 
 import app.routes.plex.admin as admin_routes
 from app import sysconfig
-from app.models import PlexLink
+from app.plex.models import PlexLink
 from app.plex.client import PlexError, PlexNotConfigured