feat(email): wire MessageIdUtil into EmailService outbound headers

[mpge]

Summary

Wires the MessageIdUtil helpers (added in #24) into EmailService so outbound ticket notifications carry canonical RFC 5322 Message-ID headers plus signed Reply-To.

Headers set

Scenario	Message-ID	In-Reply-To	References	Reply-To
Initial ticket notification	<ticket-{id}@{domain}>	—	—	reply+{id}.{hmac8}@{domain}
Reply notification	<ticket-{id}-reply-{replyId}@{domain}>	ticket root	ticket root	signed
Satisfaction survey	—	—	—	signed

The signed Reply-To routes inbound replies back to the right ticket even when the mail client strips our Message-ID / In-Reply-To chain.

Config

New EscalatedProperties.Email:

• escalated.email.domain (default: localhost) — right-hand side of Message-IDs + Reply-To
• escalated.email.inbound-secret (default: empty — Reply-To is omitted when unset, so basic threading still works without activating the inbound pipeline)

Cleanup

Removes the previous reliance on ticket.getEmailMessageId() / reply.getEmailMessageId() — these fields were never populated and yielded null Message-IDs.

Dependencies

• Stacked on feat(email): add MessageIdUtil for RFC 5322 threading + signed Reply-To #24 (feat/email-message-id). Will rebase onto main once feat(email): add MessageIdUtil for RFC 5322 threading + signed Reply-To #24 merges.

Test plan

• 5 unit tests: initial notification headers, reply-in-thread headers, inbound-secret-disabled path, satisfaction survey. Uses real MimeMessage + a mocked JavaMailSender so assertions are on actual header values.
• CI green (won't trigger against stacked base until rebased)