feat(newsletters): newsletter schema + models + renderer (Wave 3, WordPress - partial)

[mpge]

Summary

Wave 3 partial port of the newsletter engine to WordPress. Per the spec, WordPress is special-cased (WP options + WP-Cron + WP capabilities instead of the cross-framework settings table / cron tick / permission system).

• 5 new tables created by extending Escalated\Activator::create_newsletter_tables() (wired into activate() + the upgrade path)
• marketing_opt_out_at column added to escalated_contacts (via ALTER TABLE if missing)
• 5 model wrappers under includes/Models/Newsletter/ (static $wpdb helpers, same pattern as existing Models)
• Full renderer at includes/Services/Newsletter/NewsletterRenderer.php — Markdown → theme → click rewrite → pixel injection
• Two starter PHP themes under templates/newsletter_themes/ (default + branded)
• README documenting host integrator wiring (Markdown filter, themes dir filter, etc.)

Architecture notes

• Markdown is host-pluggable via the escalated_newsletter_markdown_renderer WordPress filter (Parsedown, league/commonmark, etc.).
• Themes are PHP files (not Twig/Blade) — the renderer captures their output via ob_start() / ob_get_clean(). Hosts can override the themes dir via the escalated_newsletter_themes_dir filter.
• URL generation uses home_url() so it works on subdir/subdomain WP installs.
• Schema migration uses dbDelta (the WP convention) and gates the ALTER TABLE on a column-existence check to make the upgrade idempotent.

Out of scope / follow-ups

• WP-Cron registration for the dispatcher tick (a single wp_schedule_event('escalated_every_minute', 'escalated_newsletters_dispatch') call)
• Planner / Dispatcher / Tracker services
• REST API endpoints (/escalated/n/o/{token}.gif, /c/{token}, /u/{token}, /v/{token})
• Admin pages (custom WP admin screens or via the Inertia frontend already embedded in this plugin)
• ESP webhook handlers

Reviewer notes

• Do not auto-merge. Hand back to Matthew for review.
• The create_newsletter_tables method is intentionally public static (not private) so it can be re-run from a migration command in the future without touching the activator's main activate() flow.