Send branded HTML emails and harden SMTP2GO webhooks.

Fix campaign stuck on sending under async queue, and stop UUID ValidationError when SMTP2GO tests send "Headers Unavailable".

site/messaging/README.md

@@ -26,7 +26,8 @@ SMTP2GO → **Settings → Webhooks** (email and SMS stay separate).
 | SMS events | leave unchecked |
 
 `X-Monica-Message-Id` is set on every campaign email send and is required so webhook
-events match the correct recipient row.
+events match the correct recipient row. Invalid / missing header values no longer
+500 the endpoint (SMTP2GO “Test this webhook” often sends a sample non-UUID).
 
 Beta / other hosts: swap the hostname, keep the path.