feat(inbound): scaffold InboundMessage + InboundEmailParser + InboundEmailRouter

[mpge]

Summary

Greenfield inbound-email foundation for Spring. Introduces:

• InboundMessage — transport-agnostic record capturing a normalized inbound email (from, to, subject, bodies, threading headers, attachments).
• InboundEmailParser — pluggable interface for provider-specific webhook payload normalization (Postmark, Mailgun, SES, IMAP).
• InboundEmailRouter — the resolution brain that maps an inbound message to an existing ticket via canonical Message-ID parsing + signed Reply-To verification.

Mirrors the NestJS reference and the per-framework inbound-verify PRs (Laravel #70, Rails #45, Django #42, Adonis #50, WordPress #33) + the greenfield .NET router (escalated-dotnet #23).

Resolution order

1. In-Reply-To parsed via MessageIdUtil.parseTicketIdFromMessageId — cold-start path, no DB lookup required.
2. References parsed via MessageIdUtil, each id in order.
3. Signed Reply-To on toEmail (reply+{id}.{hmac8}@...) verified via MessageIdUtil.verifyReplyTo. Forged signatures rejected with the timing-safe HMAC comparison.
4. Subject line reference tag ([{PREFIX}-...]) — legacy.

Scope

Router + DTO + interface only. Follow-up PRs:

1. Per-provider parser implementations (Postmark, Mailgun, SES)
2. InboundEmailController — POST /escalated/webhook/email/inbound
3. InboundEmailService — full process() orchestration with threading / attachments

Dependencies

• Stacked on feat(email): wire MessageIdUtil into EmailService outbound headers #25 (feat/email-service-wireup), which is stacked on feat(email): add MessageIdUtil for RFC 5322 threading + signed Reply-To #24 (feat/email-message-id). Uses MessageIdUtil + EscalatedProperties.Email. Merge order: feat(email): add MessageIdUtil for RFC 5322 threading + signed Reply-To #24 → feat(email): wire MessageIdUtil into EmailService outbound headers #25 → this PR.

Test plan

• 10 Mockito tests verify every branch:
  • Canonical In-Reply-To parse
  • References with mixed unrelated ids
  • Signed Reply-To round-trip
  • Forgery rejection (wrong secret)
  • Secret-blank skip path
  • Subject-tag match
  • No-match empty Optional
  • Null-message safeguard
  • candidateHeaderMessageIds: in-reply-to first, references after
  • candidateHeaderMessageIds: empty headers yield none
• CI green (won't trigger against stacked base until rebased)