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_overlay.py

@@ -14,7 +14,7 @@ from sqlalchemy.orm import Session
 import app.plex.sync as syncmod
 from app import sysconfig
 from app.db import engine
-from app.models import PlexItem, PlexLibrary, PlexShow
+from app.plex.models import PlexItem, PlexLibrary, PlexShow
 from app.plex.genres import hu_genre, hu_genres
 from app.plex.overlay import sync_overlay
 from app.plex.sync import _apply_item, _canonical_guid