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_cards.py

@@ -5,7 +5,7 @@ and the crossfading hero), but ONLY when the item actually has an `art_key` —
 resolve to a blank image on every card that tried to use it, so it must be null and let the client fall
 back to the poster."""
 
-from app.models import PlexItem, PlexShow
+from app.plex.models import PlexItem, PlexShow
 from app.plex.cards import movie_card, show_card