修复(#134): 促销前端展示对齐 (expires_at / billing 补 promo_discount / 类型补字段)

Squash merge of fix/134-促销前端展示对齐 (4702dee).

HIF-79/HIF-113 落地后前端有三处对齐缺失：

- promo-info.tsx 仍读 promo.end_time，但后端 SubscribeDiscountPromo
  JSON 字段是 expires_at（×2 处）
- billing.tsx 缺 Promo Discount 行，PreOrderResponse 拿到 promo_discount
  时无处可渲染
- typings.d.ts: SubscribeDiscountPromo 缺 expires_at/rule_type,
  PreOrderResponse 缺 promo_discount

本次仅修字段对齐与展示行，不动业务逻辑、不动 OrderDetail 类型 (后端
当前未透出 promo_discount)。新增 billing.promoDiscount 中英文文案。

apps/user/public/assets/locales/en-US/subscribe.json

@@ -9,6 +9,7 @@
     "originalPrice": "Original Price (Monthly)",
     "price": "Price",
     "productDiscount": "Product Discount",
+    "promoDiscount": "Promo Discount",
     "total": "Total"
   },
   "buy": "Buy",

apps/user/public/assets/locales/zh-CN/subscribe.json

@@ -9,6 +9,7 @@
     "originalPrice": "原价（按月）",
     "price": "价格",
     "productDiscount": "商品折扣",
+    "promoDiscount": "促销优惠",
     "total": "总价"
   },
   "buy": "购买",