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_collation.py

@@ -9,7 +9,8 @@ from app.auth import current_user
 from app.collation import sorted_words
 from app.db import get_db
 from app.main import app
-from app.models import PlexCollection, PlexItem, PlexLibrary, User
+from app.models import User
+from app.plex.models import PlexCollection, PlexItem, PlexLibrary
 from app.catalog.models import Channel, Subscription, Video
 from app.reader.models import Playlist, PlaylistItem
 from datetime import datetime, timezone