Use AsyncSmtpTransport.from_url to configure email?

[Nutomic]

This would be much simpler to handle, similar to the database connection which we also switched to url.

https://docs.rs/lettre/0.11.14/lettre/transport/smtp/struct.AsyncSmtpTransport.html#method.from_url

At the same time, it would make sense to enable the pool feature for connection pool, and store the email transport in OnceLock instead of building it from scratch every time.

[dessalines]

I'd be fine with this, as long as we note the breaking change.