update update_at for use

app/Services/LLM/DeepSeekService.php

@@ -120,6 +120,8 @@ class DeepSeekService
             ]);
         } else {
             $session = LlmSession::where('session_id', $sessionId)->firstOrFail();
+            $session->updated_at = now();
+            $session->save();
         }
 
         // 保存用户消息