Send the stored Reply-To on outgoing mail #47

Background

Transactional sends accept and store a replyTo (transactional/queries.ts:339, falling back to the ESP's configured reply-to). But MailInput in mail/send.ts:8-25 has no replyTo field, so the header is never sent and replies go to the From address.

Why it matters: Reply-To is defined in RFC 5322 §3.6.2. Teams send from noreply@ and expect replies to reach a support inbox, and embedding platforms pass a per-tenant reply-to and assume it's honoured.

Proposed fix

• Add replyTo?: string to MailInput and pass it to transporter.sendMail.
• Thread it through the transactional worker.
• Use the ESP-level reply-to for broadcasts and sequences.

Acceptance criteria

• A transactional send with replyTo produces a Reply-To header.
• Broadcast and sequence sends use the configured ESP reply-to when one is set.
• Tests cover both paths.