feat: add support for smtp when send email

[jonahcui]

虽然 resend 很好用，但是 smtp 还是要支持一下的，支持使用 smtp 发送邀请邮件

[crazywoola]

Please add my wechat: crazyphage, I will invite to you our developers group.

[jonahcui]

So here we're using app.config.get('xxx') to get configuration variables, but it seems like there's no logic declared in config.py to fetch the environment variables?

Thank u, I changed the config.py and submitted it.