fix: interview creation show real error msg instead of generic failure

- Backend: wrap AI call in try/catch, throw HttpException(503)
  with msg 'AI 服务暂时不可用，请稍后重试'
- Backend: AllExceptionsFilter preserve original error message
  for non-HttpException errors
- Frontend: handle string/object res.data, show statusCode
  as fallback when message is unavailable

zhiyin-app/src/pages/interview/interview.vue

@@ -242,7 +242,8 @@ const startInterview = async () => {
     } else if (checkAuth(res)) {
       return // token 过期，已清除并跳转登录
     } else {
-      const msg = res.data?.message || '创建面试失败'
+      const errMsg = typeof res.data === 'string' ? res.data : (res.data?.message || '')
+      const msg = errMsg || `创建面试失败（${res.statusCode}）`
       messages.value.push({ role: 'ai', content: msg })
     }
   } catch {