feat(coupon): add /api/v1/coupon/step proxy to pricebot

- new POST /api/v1/coupon/step, JWT-authenticated, async httpx forwards body to pricebot
- new PRICEBOT_BASE_URL / PRICEBOT_REQUEST_TIMEOUT_SEC settings (default localhost:8000)
- error handling: pricebot unreachable / 5xx -> 502 with friendly message
- tests: auth, passthrough, 5xx, unreachable, invalid json (5 cases, all pass)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.env.example

@@ -34,6 +34,13 @@ SMS_MOCK=true
 SMS_CODE_TTL_SEC=300
 SMS_SEND_INTERVAL_SEC=60
 
+# ===== Pricebot 上游 (领券业务透传目标) =====
+# 客户端调本服务的 /api/v1/coupon/step,我们透传到 pricebot-backend 的 /api/coupon/step。
+# 本地开发用 localhost:8000。生产部署改成内网地址(如 http://pricebot.internal:8000)。
+PRICEBOT_BASE_URL=http://localhost:8000
+# 领券单帧最多 wait 6s,加网络往返,30s 兜底
+PRICEBOT_REQUEST_TIMEOUT_SEC=30
+
 # ===== CORS =====
 # 逗号分隔,生产留空(只让 app 调,不开放 web)。本地开发可加 http://localhost:5173 之类
 CORS_ALLOW_ORIGINS=