Add smtp to diagnostics

[hugosxm]

Hi ! I battled 1H with vaultwarden because the smtp was dead, it tries to send an email every minute because of this job :

[vaultwarden::api::core::two_factor][DEBUG] Sending notifications for incomplete 2FA logins

and with the failed smtp it open a db connection...left it hanging... then the next minute again... till max connection is reached and mariadb refuses it.

Long story short : having a smtp healthcheck in the diagnostics web page of vaultwarden would be cool :)

Have a nice day !

[BlackDex]

We can't just sent random emails everytime a user opens the diagnostics page.
We could maybe check if we can connect, but that is about it. That will not help with anything beyond that.

There is however, in the Settings page an option to test SMTP by sending an email.

[hugosxm]

Yeah I was thinking about a simple way to get a HELLO from the smtp server, that would solve my case at least.

I was aware of the test SMTP button, but as the main error was DB related at first it did not fiddle with smtp settings