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/data.js

@@ -35,6 +35,7 @@ import {
   AUDIT_LOG_FILE,
   WEBHOOKS_FILE,
   STUDY_SESSIONS_FILE,
+  NEWSLETTER_ISSUES_FILE,
   EMPTY_HIT_STATS,
   EMPTY_VISITOR_STATS,
   DEFAULT_REPLY_TEMPLATES,
@@ -1673,3 +1674,27 @@ export function loadQrCodesFromDisk() {
       state.qrScans = []
     })
 }
+
+// ── Newsletter Issues ────────────────────────────────────────────────────────
+
+export function queueNewsletterIssuesWrite() {
+  state.newsletterIssuesWritePromise = state.newsletterIssuesWritePromise
+    .then(async () => {
+      await mkdir(DATA_DIR, { recursive: true })
+      await writeFile(NEWSLETTER_ISSUES_FILE, JSON.stringify({ issues: state.newsletterIssues }, null, 2), 'utf8')
+    })
+    .catch(err => {
+      console.error('[newsletter-issues] failed to write:', err)
+    })
+}
+
+export function loadNewsletterIssuesFromDisk() {
+  return readFile(NEWSLETTER_ISSUES_FILE, 'utf8')
+    .then(raw => {
+      const parsed = JSON.parse(raw)
+      state.newsletterIssues = Array.isArray(parsed?.issues) ? parsed.issues : []
+    })
+    .catch(() => {
+      state.newsletterIssues = []
+    })
+}