Show build commit/branch in the footer

Single-line addition next to the existing Pelican/Python credit,
using the same '·' delimiter as the header toggles. Only renders when
SITE_COMMIT is set (i.e. built via the CI workflow, not a plain local
build).

templates/base.html

@@ -124,6 +124,9 @@
   <footer id="contentinfo" class="body">
     <address id="about" class="vcard body">
       Proudly powered by <a href="http://getpelican.com/">Pelican</a>, which takes great advantage of <a href="http://python.org">Python</a>.
+      {% if SITE_COMMIT %}
+      <span class="header-delimiter">·</span> Build <a href="https://git.haemka.in/hmk/haemka.de/commit/{{ SITE_COMMIT }}">{{ SITE_COMMIT }}</a> ({{ SITE_BRANCH }})
+      {% endif %}
     </address><!-- /#about -->
   </footer><!-- /#contentinfo -->