feat(compare): 美团比价前用券透传端点 /intent/precoupon/step (#41)

## 背景
客户端在美团源比价的**意图识别前**先循环调本端点用券。app-server 只做透传壳(决策在 pricebot-backend)。

## 改动
- 新增 `POST /api/v1/intent/precoupon/step` → 透传到 pricebot-backend `/api/intent/precoupon/step`
- 同 `/intent/step` 透传壳: 不鉴权、不做 schema 校验, device_id 透传区分设备

## 测试
- 链路自测(:8770 → :8000) + 真机实测用券成功

## 关联(跨三仓库, 同名分支 `feat/meituan-precoupon`)
- WonderableAI/pricebot-backend
- WonderableAI/shaguabijia-app-server
- WonderableAI/shaguabijia-app-android
**改协议字段需三仓库一起改。**

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: xianze <ze@192.168.0.128>
Reviewed-on: #41
Co-authored-by: zhangxianze <zhangxianze@wonderable.ai>
Co-committed-by: zhangxianze <zhangxianze@wonderable.ai>

app/api/v1/compare.py

@@ -102,6 +102,17 @@ async def intent_step(request: Request) -> dict[str, Any]:
     return await _passthrough(request, "/api/intent/step")
 
 
+@router.post(
+    "/intent/precoupon/step",
+    summary="外卖比价 Phase 0 意图识别前先用券 (透传到 pricebot, 仅美团源)",
+)
+async def intent_precoupon_step(request: Request) -> dict[str, Any]:
+    # 美团源平台『意图识别前先用券』多帧循环: 客户端在调 /intent/recognize 之前先循环
+    # 调本端点到 done(continue=false)。订单页底部有『点击使用X红包』就自动选最大免费券
+    # 用上, 已用券/无券则首帧秒过。与 /intent/step 同属 intent 域, 复用同一透传壳。
+    return await _passthrough(request, "/api/intent/precoupon/step")
+
+
 @router.post("/price/step", summary="外卖比价 Phase 2 步进 (透传到 pricebot)")
 async def price_step(request: Request) -> dict[str, Any]:
     return await _passthrough(request, "/api/price/step")