feat(email): wire MessageIdUtil into EmailTemplates + signed Reply-To

[mpge]

Summary

Refactors EmailTemplates.NewTicket / TicketReply / SlaBreachAlert to delegate Message-ID generation to MessageIdUtil (added in #21) so the format matches the canonical NestJS reference, and adds a signed Reply-To header so inbound provider webhooks can verify ticket identity.

Format change

	Before	After
Anchor	<{ticket.Reference}@escalated>	<ticket-{id}@{domain}>
Reply	<{ticket.Reference}-reply-{reply.Id}@escalated>	<ticket-{id}-reply-{replyId}@{domain}>
SLA	<{ticket.Reference}-sla-{breachType}@escalated>	In-Reply-To + References point at ticket root

The hardcoded @escalated pseudo-domain is replaced with the new EmailOptions.Domain config value. Every template now takes an EmailOptions parameter so the domain + secret don't have to be baked into call sites.

Config

EscalatedOptions.Email.Domain        // default: "localhost"
EscalatedOptions.Email.InboundSecret // default: "" — Reply-To skipped

Host apps with the default DI wiring resolve IOptions<EscalatedOptions> and pass options.Value.Email to the templates.

EmailMessage.ReplyTo

New nullable ReplyTo property on EmailMessage holds the signed reply+{id}.{hmac8}@{domain} address (or null when InboundSecret is blank). Inbound provider webhooks verify via MessageIdUtil.VerifyReplyTo to route replies back to their ticket even when clients strip the Message-ID chain.

Dependencies

• Stacked on feat(email): add MessageIdUtil for RFC 5322 threading + signed Reply-To #21 (feat/email-message-id). Will rebase onto main once feat(email): add MessageIdUtil for RFC 5322 threading + signed Reply-To #21 merges.

Test plan

• 6 xUnit tests cover the Message-ID format, In-Reply-To / References headers, signed Reply-To presence / absence, and the default-option values
• CI green (won't trigger against stacked base until rebased)