feat(web): show git SHA in footer for post-deploy verification

Dockerfile takes SOURCE_COMMIT as a build arg and bakes it into the
image as GIT_COMMIT. Coolify sets SOURCE_COMMIT on every deploy, so
the value in the footer changes with each successful push → build.

ARG is placed after COPY . so only a thin final layer rebuilds when
the SHA changes; pip install stays cached. Outside Coolify the
default is "dev" and the footer renders "build dev".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/templates/_layout.html

@@ -31,5 +31,9 @@
 </main>
 <footer class="text-center text-xs text-slate-500 py-6">
     Mit ♥ programmiert für Annika
+    {% if git_commit_short %}
+    <span class="mx-1">·</span>
+    <span title="{{ git_commit }}">build {{ git_commit_short }}</span>
+    {% endif %}
 </footer>
 {% endblock %}