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/query.py

@@ -10,7 +10,7 @@ from sqlalchemy import and_, case, func, or_, select, type_coerce
 from sqlalchemy.dialects.postgresql import JSONB
 from sqlalchemy.orm import Session
 
-from app.models import PlexItem, PlexLibrary
+from app.plex.models import PlexItem, PlexLibrary
 from app.plex.genres import hu_genre
 
 TS_CONFIG = "public.unaccent_simple"