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_watch_reconcile.py

@@ -7,7 +7,8 @@ import pytest
 from sqlalchemy import select
 
 from app import sysconfig
-from app.models import PlexItem, PlexLibrary, PlexLink, PlexState, User
+from app.models import User
+from app.plex.models import PlexItem, PlexLibrary, PlexLink, PlexState
 from app.plex import watch_sync