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_sync_txn.py

@@ -7,7 +7,7 @@ from sqlalchemy import select
 
 from app.plex import people, sync
 from app.plex.client import PlexError
-from app.models import PlexItem, PlexLibrary, PlexPerson
+from app.plex.models import PlexItem, PlexLibrary, PlexPerson
 
 
 class _FakePlex: