feat: 新增52号发际线接口并限制GPU worker来源IP

保留5号历史三档实现，新增中下庭高度定位、扶正裁切、矩形叠加与30案例对比页。

worker仅允许本机和配置白名单访问；网关不增加IP限制。空图片URL字段改写限定为52号，保持旧接口兼容。同步接口、裁切与部署文档。

验证：113项测试通过；1项历史数值基线失败，在未修改的a3fddbb版本复现。浏览器验证5/52及30案例对比页通过。

gateway/forward.py

@@ -88,6 +88,8 @@ def rewrite_base64_to_url(
     obj: Any,
     public_base_url: str,
     static_dir: str,
+    *,
+    rewrite_null_images: bool = False,
 ) -> Any:
     """递归遍历响应 JSON，将所有 *_base64 字段改写为 *_url。
 
@@ -95,10 +97,16 @@ def rewrite_base64_to_url(
     - 支持 data: URI 和原始 base64 两种格式
     - 解码 base64 → 保存到 static_dir/{uuid}.png
     - 删除 *_base64 字段，新增 *_url 字段指向公网 URL
+    - 接口52显式开启 rewrite_null_images，空图片也返回 *_url: null；旧接口保持原空字段
     """
     if isinstance(obj, dict):
         new_dict: Dict[str, Any] = {}
         for key, value in obj.items():
+            if rewrite_null_images and key.endswith("_base64") and value is None:
+                # Deprecated/optional image fields must keep their external
+                # *_url names even when no image is generated.
+                new_dict[key[:-7] + "_url"] = None
+                continue
             if key.endswith("_base64") and isinstance(value, str):
                 img_bytes = _decode_base64_value(value)
                 if img_bytes is not None:
@@ -118,10 +126,12 @@ def rewrite_base64_to_url(
                     logger.warning("字段 %s 的值无法解码为图片，保留", key)
                     new_dict[key] = value
             else:
-                new_dict[key] = rewrite_base64_to_url(value, public_base_url, static_dir)
+                new_dict[key] = rewrite_base64_to_url(
+                    value, public_base_url, static_dir, rewrite_null_images=rewrite_null_images)
         return new_dict
     elif isinstance(obj, list):
-        return [rewrite_base64_to_url(item, public_base_url, static_dir) for item in obj]
+        return [rewrite_base64_to_url(item, public_base_url, static_dir,
+                                     rewrite_null_images=rewrite_null_images) for item in obj]
     else:
         return obj
 
@@ -248,7 +258,9 @@ async def proxy_request(request: Request, path: str) -> JSONResponse:
                 )
 
             # 递归改写 base64 图片字段
-            rewritten = rewrite_base64_to_url(worker_json, public_base_url, static_dir)
+            rewritten = rewrite_base64_to_url(
+                worker_json, public_base_url, static_dir,
+                rewrite_null_images=path == "/api/v1/hairline/generate-52")
 
             return JSONResponse(
                 status_code=resp.status_code,