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/config.js

@@ -74,6 +74,7 @@ export const CALENDAR_EVENTS_FILE = path.join(DATA_DIR, 'calendar-events.json')
 export const AUDIT_LOG_FILE = path.join(DATA_DIR, 'audit-log.json')
 export const WEBHOOKS_FILE = path.join(DATA_DIR, 'webhooks.json')
 export const STUDY_SESSIONS_FILE = path.join(DATA_DIR, 'study-sessions.json')
+export const NEWSLETTER_ISSUES_FILE = path.join(DATA_DIR, 'newsletter-issues.json')
 export const MAX_EPISODE_SCRIPT_LENGTH = 200_000 // ~150k words, well beyond any sermon
 
 // Seafile integration for Nuggets episode uploads