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_empty_listing_guards.py

@@ -12,7 +12,7 @@ contradicts the remote's own count is not believed either.
 import pytest
 from sqlalchemy import select
 
-from app.models import PlexCollection, PlexItem, PlexLibrary
+from app.plex.models import PlexCollection, PlexItem, PlexLibrary
 from app.catalog.models import Channel, Video
 from app.reader.models import Playlist, PlaylistItem
 from app.plex import sync as plex_sync