Configure email delivery in production using environment variables

[harisadam]

Looks like Fizzy currently defaults to Sendmail, this update enables developers to switch to SMTP when needed via environment variables.

[arnsteinsm]

Hey! Just wanted to add a quick note: before I found this PR, I ended up doing almost the same SMTP setup in my own fork so I could run Fizzy self-hosted.

I’ve tested the env-based SMTP approach on Coolify (ARM server) using Mailgun, and it works very well.

Thanks for putting it together!

[kevinmcconnell]

Note that we're currently setting specific SMTP settings in fizzy-saas, which would be overwritten by the settings provided here.

So before we can merge this, we need to resolve that situation. Either fizzy-sass needs to use the same env vars, or maybe invert the order that these environment files get applied so that fizzy-sass can take priority.

cc @jorgemanrubia

[harisadam]

@kevinmcconnell This might be a better approach and make this PR irrelevant: https://github.com/basecamp/fizzy/pull/1958