fix: 素材 API 错误信息中文映射（同 Seedance 模式）

AssetsAPIError 加 user_message，按 code/关键词映射中文提示，用户不再看到英文错误

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/apps/generation/views.py

@@ -2726,10 +2726,8 @@ def _assets_api_call(func, *args, **kwargs):
         return result, None
     except AssetsAPIError as e:
         logger.warning('Assets API error: %s', e)
-        # Surface readable message for known parameter errors
-        msg = e.api_message if hasattr(e, 'api_message') else str(e)
         return None, Response(
-            {'error': 'assets_api_error', 'message': msg},
+            {'error': 'assets_api_error', 'message': e.user_message},
             status=status.HTTP_502_BAD_GATEWAY,
         )
     except Exception as e:

backend/utils/assets_client.py

@@ -21,12 +21,41 @@ HOST = 'open.volcengineapi.com'
 PROJECT_NAME = 'int_dev_Airlabs'
 
 
+_ASSETS_ERROR_MESSAGES = {
+    'ConfigError': '素材服务未配置，请联系管理员',
+    'RequestError': '素材服务暂时不可用，请稍后重试',
+    'InvalidParameter': '素材参数无效，请检查输入',
+    'NotFound': '素材不存在或已被删除',
+    'NotExist': '素材不存在或已被删除',
+    'InternalError': '素材服务异常，请稍后重试',
+    'Forbidden': '没有权限操作该素材',
+    'RateLimitExceeded': '操作过于频繁，请稍后重试',
+}
+
+_ASSETS_MESSAGE_KEYWORDS = {
+    'dimension': '图片尺寸不符合要求（宽高需在 300~6000 像素之间）',
+    'size': '文件大小超出限制',
+    'format': '不支持的文件格式',
+    'not found': '素材不存在或已被删除',
+    'permission': '没有权限操作该素材',
+}
+
+
 class AssetsAPIError(Exception):
     """Raised when the Assets API returns an error."""
     def __init__(self, code, message, status_code=400):
         self.code = code
         self.api_message = message
         self.status_code = status_code
+        # 中文友好提示
+        friendly = _ASSETS_ERROR_MESSAGES.get(code)
+        if not friendly:
+            msg_lower = (message or '').lower()
+            for keyword, hint in _ASSETS_MESSAGE_KEYWORDS.items():
+                if keyword in msg_lower:
+                    friendly = hint
+                    break
+        self.user_message = friendly or '素材操作失败，请稍后重试'
         super().__init__(f'[{code}] {message}')