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>

package.json

@@ -1,7 +1,7 @@
 {
   "name": "siteforge",
   "private": true,
-  "version": "2.2.4",
+  "version": "2.3.0",
   "type": "module",
   "scripts": {
     "dev": "vite",