fix: guard _meta.html against missing post variable

Calendar templates extend post/index.html but don't provide a
post context variable. Wrap post-specific meta in an if guard
so pages without post still get basic meta from meta_base.html.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

suma_browser/templates/_types/post/_meta.html

@@ -2,6 +2,9 @@
 {# Context expected:
    site, post, request
 #}
+{% if post is not defined %}
+  {% include 'social/meta_base.html' %}
+{% else %}
 
 {# Visibility → robots #}
 {% set is_public = (post.visibility == 'public') %}
@@ -122,3 +125,4 @@
 <script type="application/ld+json">
 {{ jsonld | tojson }}
 </script>
+{% endif %}