feat: add WebSub support for real-time feed updates

Advertise WebSub hub (websubhub.com) in three discovery layers:
- HTML <link rel="hub"> in page head
- <atom:link rel="hub"> in RSS feed
- "hubs" array in JSON Feed 1.1

Notify hub after each Eleventy build so subscribers receive
push updates when new content is published.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

_includes/layouts/base.njk

@@ -65,6 +65,7 @@
   <link rel="token_endpoint" href="{{ site.url }}/auth/token">
   <link rel="micropub" href="{{ site.url }}/micropub">
   <link rel="microsub" href="{{ site.url }}/microsub">
+  <link rel="hub" href="https://websubhub.com/hub">
   <link rel="webmention" href="https://webmention.io/{{ site.webmentions.domain }}/webmention">
   <link rel="pingback" href="https://webmention.io/{{ site.webmentions.domain }}/xmlrpc">