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/state.js

@@ -96,6 +96,10 @@ export const state = {
   emailSettings: { signature: 'Grace and peace,\nVerse by Verse with Nate' },
   emailSettingsWritePromise: Promise.resolve(),
 
+  // newsletterIssues: Array<{ id, resendBroadcastId, slug, title, subject, description, html, sentAt, publishedAt, public }>
+  newsletterIssues: [],
+  newsletterIssuesWritePromise: Promise.resolve(),
+
   calendarEvents: [],
   calendarEventsWritePromise: Promise.resolve(),