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_plaintext.py

@@ -12,7 +12,7 @@ from sqlalchemy import select
 from sqlalchemy.orm import Session
 
 from app.db import engine
-from app.models import PlexCollection, PlexItem, PlexLibrary, PlexShow
+from app.plex.models import PlexCollection, PlexItem, PlexLibrary, PlexShow
 from app.plex.detail import rich_detail
 from app.plex.overlay import sync_overlay
 from app.plex.plaintext import plain_text