Use the sender's domain in Message-ID instead of SendLit's host #49

Background

generateRfcMessageId() (utils/rfc-message-id.ts:14-22) always uses SendLit's host (getSiteUrl(), or sendlit.local as a fallback) on the right-hand side of Message-ID, whatever the From domain is.

Why it matters: RFC 5322 §3.6.4 expects a globally unique id, and the convention is to use the sender's domain. A domain that doesn't match From (or localhost/sendlit.local when self-hosted) is a weak spam signal, and it exposes the platform host on white-labelled sends.

Proposed fix

• Change the function to generateRfcMessageId(fromDomain).
• Keep the current host only as a fallback.

Acceptance criteria

• Sends from [email protected] get a Message-ID ending in @acme.com.
• Correlation via outbound_messages.rfcMessageId still works.
• A unit test covers it.