feat(接口2/7): hair_style改为多选复选框，新增接口7测试页，网关注册v2路由

- 接口2/7测试页：发型从单选下拉框改为多选复选框+全选/全不选，提交时逗号分隔
- 新建static/test_interface7.html（基于接口2测试页，紫色主题，调用/api/v1/hair/grow-v2）
- gateway/app.py：注册/api/v1/hair/grow-v2代理路由 + 首页索引
- integration.html：新增接口7完整文档卡片（入参/出参/测试页链接）

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

gateway/app.py

@@ -196,6 +196,7 @@ async def index():
             "if3_hair_grow_b": "/static/test_interface3.html",
             "if4_features": "/static/test_interface4.html",
             "if5_hairline": "/static/test_interface5.html",
+            "if7_hair_grow_v2": "/static/test_interface7.html",
         },
     }
 
@@ -320,3 +321,9 @@ async def face_features(
 async def hairline_generate(request: Request):
     """接口5：发际线PNG生成"""
     return await _proxy(request, "/api/v1/hairline/generate")
+
+
+@app.post("/api/v1/hair/grow-v2", tags=["生发"])
+async def hair_grow_v2(request: Request):
+    """接口7：C端生发 v2（add_hair2 工作流）"""
+    return await _proxy(request, "/api/v1/hair/grow-v2")