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>

src/components/GlobalSearch.tsx

@@ -8,6 +8,7 @@ const TYPE_LABEL: Record<SearchResultType, string> = {
   question: 'Q&A',
   resource: 'Download',
   series: 'Series',
+  newsletter: 'Newsletter',
 }
 
 const TYPE_ICON: Record<SearchResultType, string> = {
@@ -15,6 +16,7 @@ const TYPE_ICON: Record<SearchResultType, string> = {
   question: '❓',
   resource: '📄',
   series: '📚',
+  newsletter: '✉️',
 }
 
 interface Props {
@@ -57,6 +59,9 @@ export function GlobalSearch({ query, setQuery, results }: Props) {
     setQuery('')
     if (result.href.startsWith('http')) {
       window.open(result.href, '_blank', 'noreferrer')
+    } else if (result.type === 'newsletter') {
+      // Newsletter issues are server-rendered outside the SPA (for social-share OG tags), so use a full navigation.
+      window.location.href = result.href
     } else {
       navigate(result.href)
     }