Add a newsletter archive so past issues can be shared and found; v2.3.0

Web-hosted copies of episode newsletters, published from Resend broadcasts
tagged with the "New Episode" topic. Gives Facebook (and anyone else) a
proper link to share instead of raw HTML email, and makes issues findable
via site search and Google.

- Admin "Newsletter Archive" tab lists sent Resend broadcasts, flags ones
  tagged "New Episode", and publishes them with one click (pulls the HTML
  straight from Resend's API)
- Public /newsletter archive index and /newsletter/:slug issue pages,
  server-rendered with per-issue Open Graph tags for social previews
- Unsubscribe merge-tag stripped from the web copy; archive + subscribe
  links appended
- Issues added to sitemap.xml and the on-site search index

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

server/routes/public.js

@@ -106,6 +106,11 @@ export function register(app) {
     for (const ep of highlights) {
       if (ep.id) urls.push(`${root}/episodes/${encodeURIComponent(ep.id)}`)
     }
+    // Newsletter archive + published issues
+    urls.push(`${root}/newsletter`)
+    for (const issue of state.newsletterIssues) {
+      if (issue.slug && issue.public !== false) urls.push(`${root}/newsletter/${encodeURIComponent(issue.slug)}`)
+    }
 
     const xml = [
       '<?xml version="1.0" encoding="UTF-8"?>',