feat(接口4): 精简输出为固定6个英文字段，缩短提示词

- face_features.py：_PROMPT 只问6项特征（+有无人脸），analyze_features
  只返回6个英文字段；无人脸返回 None（不再依赖 has_face 在外部判定）
- gateway/app.py：无脸判定改为 feats is None，去掉 has_face import
- app.py / docs / test_interface4.html：Swagger/文档/示例/测试页同步

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

gateway/app.py

@@ -291,7 +291,7 @@ async def face_features(
             })
 
     from fastapi.concurrency import run_in_threadpool
-    from face_features import analyze_features, has_face
+    from face_features import analyze_features
 
     try:
         feats = await run_in_threadpool(analyze_features, img_bytes, image_url)
@@ -302,7 +302,7 @@ async def face_features(
             "request_id": f"gw-{_uuid.uuid4().hex[:8]}", "data": None,
         })
 
-    if not has_face(feats):
+    if feats is None:
         return JSONResponse(status_code=200, content={
             "code": 1001, "message": "无法识别人像",
             "request_id": f"gw-{_uuid.uuid4().hex[:8]}", "data": None,