feat: Bake build metadata (git tag, commit, date) into OCI image and display in web UI

Store git tag, commit hash, and build date as OCI-standard labels and a
build_meta.json file inside the Docker image. The Flask app reads this at
startup and displays version info in all page footers. Adds /version JSON
endpoint for programmatic access. Falls back to dev@local outside Docker.

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

templates/adults.html

@@ -647,7 +647,7 @@
     {% set rt = get_render_time() %}
     <div class="footer"
         onclick="document.getElementById('perf-details').style.display = (document.getElementById('perf-details').style.display === 'block' ? 'none' : 'block')">
-        render time: {{ rt.total }}s
+        {{ build_meta.tag }}@{{ build_meta.commit }} | render time: {{ rt.total }}s
         <div id="perf-details" class="perf-breakdown">
             {{ rt.breakdown }}
         </div>