On each scene change (where it already signals /hints/puzzle_start), the
npc_engine now also POSTs the active SCENE_* to the voice gateway
/game/step?scene=… (best-effort, esp_http_client, Bearer token). The
phone NPCs then disguise THIS scene's hint automatically during a game.
Gateway URL/token hardcoded in main.c like the hints URL (-> NVS later).
clement