Add an admin-only server log viewer to the web UI

The root logger previously had no handler at all, so every module's
logger.info() call (user creation, claims, password resets, ...) was
silently dropped, not just unviewable. Adds a RotatingFileHandler
writing into the existing /data volume so log content also survives
container restarts/redeploys, plus /admin/logs (tail + line-count
picker + full-file download) alongside the existing Users & Frames
admin page.

server/app/templates/admin.html

@@ -4,6 +4,8 @@
 {% block page_title %}Administration{% endblock %}
 
 {% block content %}
+  {% include "_admin_tabs.html" %}
+
   {% if notice %}<div class="status ok">{{ notice }}</div>{% endif %}
   {% if error %}<div class="status err">{{ error }}</div>{% endif %}