Function seed_image_cache_settings_from_env 

pub(crate) async fn seed_image_cache_settings_from_env(
    client: &Client,
) -> Result<(), AppError>

启动时用 IMAGE_DISK_CACHE_MAX_MB / _MAX_AGE_HOURS 环境变量播种。

语义与 seed_backup_settings_from_env 一致。