refactor(接口3): 简化为只需划线图一张，去掉 original + best_hairline

按需求方意见——B端只需上传一张已画好发际线的图，用不着原图：
- 入参去掉 original_image_*，只保留 marked_image_*(三选一)
- 输出去掉 best_hairline_image_url，只返回 hair_growth_image_url + hairline_type
- ComfyUI 输入图改用 marked 划线图原样(add_hair.json 本就是"画了线的照片"，
  提示词清除黑线)；检测路径只用于建遮罩，不再重画干净线/不需对齐原图
- service.generate_grow_b 签名改 (marked_bgr) 单参
- 同步文档：接口文档/接口3技术方案/网关映射表(去掉接口3 best_hairline 行)
- 测试更新：grow-b 只传 marked，断言无 best_hairline 字段，44全绿

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/网关-开发任务书.md

@@ -133,8 +133,7 @@ curl -s http://127.0.0.1:8080/gateway-health   # 200
    | 1 | `/api/v1/face/measure` | `annotated_image_base64` | `annotated_image_url` | data 顶层 |
    | 2 | `/api/v1/hair/grow` | `results[].image_base64` | `results[].image_url` | **数组元素** |
    | 2 | 〃 | `results[].grown_image_base64` | `results[].grown_image_url` | **数组元素**（可空） |
-   | 3 | `/api/v1/hair/grow-b` | `best_hairline_image_base64` | `best_hairline_image_url` | data 顶层 |
-   | 3 | 〃 | `hair_growth_image_base64` | `hair_growth_image_url` | data 顶层（可空） |
+   | 3 | `/api/v1/hair/grow-b` | `hair_growth_image_base64` | `hair_growth_image_url` | data 顶层（可空） |
    | 4 | `/api/v1/face/features` | —（无图片字段，原样透传）| — | — |
    | 5 | `/api/v1/hairline/generate` | `hairline_images[].image_base64` | `hairline_images[].image_url` | **数组元素** |