feat: add configurable SMTP HELO hostname (#2146)

* feat: add configurable SMTP HELO hostname

Allow the SMTP HELO/EHLO hostname to be configured separately from
the SMTP server hostname.

This is useful when the SMTP server requires clients to identify
themselves with a fully qualified hostname different from the server
address.

* chore: remove vendored dependency changes

* Bump go-pkgz/notify to v1.4.0 and document SMTP_HELO_HOST

The HELOHost field lands in go-pkgz/notify v1.4.0, so the branch needs the
bump to compile; v1.3.0 in master has no such field. The example module is
tidied alongside, as any change to backend/go.mod requires.

Documents the parameter in the parameters table and, separately, in the email
setup page: what it does, that leaving it unset keeps the previous `localhost`
greeting, and the case it exists for, a relay refusing the greeting under
Postfix `reject_non_fqdn_helo_hostname`.

Also records the current limit: verification emails for email authentication
go through go-pkgz/auth's own sender, which has no equivalent setting, so the
greeting there is unchanged.

* Bump go-pkgz/auth to v2.2.0 and apply SMTP_HELO_HOST to verification email

The verification email sender had no way to set the greeting, so a relay that
refuses the HELO would accept notifications and still reject sign-in emails.
EmailParams gains HELOHost in go-pkgz/auth v2.2.0, so the same SMTP_HELO_HOST
now drives both paths.

The example module is tidied alongside, as any change to backend/go.mod
requires.

---------

Co-authored-by: oli <someone@somewhere.tld>
Co-authored-by: Dmitry Verkhoturov <paskal.07@gmail.com>

compose-dev-backend.yml

@@ -49,6 +49,7 @@ services:
       - NOTIFY_EMAIL_FROM
       - ADMIN_SHARED_EMAIL
       - SMTP_HOST
+      - SMTP_HELO_HOST
       - SMTP_USERNAME
       - SMTP_PASSWORD
       - SMTP_PORT