feat(voice): /game/call — runtime rings NPC

The runtime triggers a scenario phone call: POST /game/call?number=18
[&step=STEP_X] rings the PLIP handset with the chosen NPC (resolved via
the boards registry) and sets the current step so the NPC disguises that
step's hint. Completes the game->phone trigger path (chantier #2).

tools/zacus-gateway/main.py

@@ -2248,3 +2248,31 @@ async def game_step(request: Request, step: str | None = None,
     cur = getattr(request.app.state, "current_step", INITIAL_STEP)
     return {"step": cur, "scene": STEP_SCENE.get(cur or "", ""),
             "has_hints": bool(SCENE_HINTS.get(STEP_SCENE.get(cur or "", ""), {}))}
+
+
+@app.post("/game/call")
+async def game_call(request: Request, number: str, board: str = "plip",
+                    step: str | None = None,
+                    _: None = Depends(require_token)) -> dict:
+    """Make a phone (board) ring with a scenario NPC. The runtime calls this when
+    the scenario reaches a 'phone rings' event: it rings the handset with the
+    chosen number and (optionally) sets the current step so the NPC disguises
+    THAT step's hint. POST /game/call?number=18[&board=plip][&step=STEP_X]."""
+    if number not in PHONE_DIRECTORY:
+        raise HTTPException(404, f"unknown phone number '{number}'")
+    if step:
+        if STEP_SCENE and step not in STEP_SCENE:
+            raise HTTPException(404, f"unknown step '{step}'")
+        request.app.state.current_step = step
+    route = resolve_board_route(board)
+    http: httpx.AsyncClient = request.app.state.http
+    try:
+        r = await http.get(f"{route.base_url}/debug/ring", params={"number": number}, timeout=5.0)
+        r.raise_for_status()
+    except Exception as exc:  # noqa: BLE001
+        raise HTTPException(502, f"ring failed on {board} ({route.base_url}): {type(exc).__name__}") from exc
+    logging.info("game call: ring %s on %s (number %s, step %s)",
+                 PHONE_DIRECTORY[number].get("label", number), board, number,
+                 getattr(request.app.state, "current_step", None))
+    return {"ok": True, "ringing": number, "npc": PHONE_DIRECTORY[number].get("label"),
+            "board": board, "step": getattr(request.app.state, "current_step", None)}