feat(newsletters): newsletter entities + renderer (Wave 3, Spring - partial)

[mpge]

Summary

Wave 3 partial port of the newsletter engine to Spring Boot / JPA. Entities + renderer + themes. DB-bound services are a follow-up.

• 5 JPA entities under dev/escalated/models/newsletter/
• Contact.marketingOptOutAt field added
• services/newsletter/NewsletterRenderer.java — full renderer (Markdown → theme → click rewrite → pixel injection)
• Two starter themes under resources/templates/escalated/newsletter_themes/ using {{key}} / {{{key}}} substitution
• README usage example

Architecture notes

• No Flyway/Liquibase migration committed — schema is auto-derived by Hibernate when spring.jpa.hibernate.ddl-auto=update, or hosts using a migration tool generate the SQL from these @Entity classes.
• Markdown is host-pluggable via NewsletterRenderer.Options.markdownToHtml (recommended: flexmark-java or commonmark-java).
• Theme rendering uses simple {{key}} / {{{key}}} substitution — no Thymeleaf dependency for the renderer (themes still live under resources/templates/ so Thymeleaf-using hosts can re-render through their own view layer).
• JPA conventions — extends existing BaseEntity for id / createdAt / updatedAt (NewsletterDelivery does not extend it because it omits updatedAt for write-volume reasons).

Out of scope / follow-ups

• Flyway / Liquibase migration script
• Planner / Dispatcher / Tracker services using JpaRepositorys
• Spring MVC controllers (admin CRUD + public tracking/unsubscribe/view-in-browser + ESP webhooks)
• @Scheduled task for the dispatcher tick

Reviewer notes

• Do not auto-merge. Hand back to Matthew for review.