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/app/plex/cards.py

@@ -5,7 +5,7 @@ router. Pure functions — no DB, no request state."""
 
 from typing import NamedTuple
 
-from app.models import PlexItem, PlexShow, PlexState
+from app.plex.models import PlexItem, PlexShow, PlexState
 
 
 class ShowState(NamedTuple):