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.js

@@ -29,6 +29,7 @@ import {
   loadWebhooksFromDisk,
   loadStudySessionsFromDisk,
   queueStudySessionsWrite,
+  loadNewsletterIssuesFromDisk,
   createBackupSnapshot,
   refreshContentCaches,
   queueHitStatsWrite,
@@ -70,6 +71,7 @@ import { startNuggetsSeafileWatcher } from './server/nuggets-seafile-watcher.js'
 import { register as registerFeed } from './server/routes/feed.js'
 import { register as registerQrCodes } from './server/routes/qr-codes.js'
 import { register as registerBiblePassage } from './server/routes/bible-passage.js'
+import { register as registerNewsletter } from './server/routes/newsletter.js'
 import { register as registerPublic } from './server/routes/public.js'
 
 const app = express()
@@ -120,6 +122,7 @@ registerNuggetsHostUpload(app)
 registerFeed(app)
 registerQrCodes(app)
 registerBiblePassage(app)
+registerNewsletter(app)
 
 // Hit-counting middleware (must come before public routes)
 app.use((req, res, next) => {
@@ -168,6 +171,7 @@ Promise.all([
   loadAuditLogFromDisk(),
   loadWebhooksFromDisk(),
   loadStudySessionsFromDisk(),
+  loadNewsletterIssuesFromDisk(),
   refreshContentCaches(),
 ])
   .catch(err => {