fix: 将52号接口范围限定为GPU worker

撤回52号网关路由、图片字段改名和mock改动，gateway目录恢复a3fddbb。worker保持base64和null响应，更新接口说明和回归测试。

验证：worker API、IP限制与网关兼容性共54项测试通过。

gateway/forward.py

@@ -88,8 +88,6 @@ 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。
 
@@ -97,16 +95,10 @@ 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:
@@ -126,12 +118,10 @@ 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, rewrite_null_images=rewrite_null_images)
+                new_dict[key] = rewrite_base64_to_url(value, public_base_url, static_dir)
         return new_dict
     elif isinstance(obj, list):
-        return [rewrite_base64_to_url(item, public_base_url, static_dir,
-                                     rewrite_null_images=rewrite_null_images) for item in obj]
+        return [rewrite_base64_to_url(item, public_base_url, static_dir) for item in obj]
     else:
         return obj
 
@@ -258,9 +248,7 @@ 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,
-                rewrite_null_images=path == "/api/v1/hairline/generate-52")
+            rewritten = rewrite_base64_to_url(worker_json, public_base_url, static_dir)
 
             return JSONResponse(
                 status_code=resp.status_code,