fix: use kpi dict from _kpi_for_period instead of removed local vars

The refactor to _kpi_for_period() removed the local variables but
the return dict still referenced them. Replace with the kpi dict.

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

blueprints/admin/routes_user_insights.py

@@ -1225,12 +1225,7 @@ def _tab_overview(db, start_date, days):
 
     return {
         'filter_type': filter_type,
-        'kpi': {
-            'active_users': active_users,
-            'sessions': total_sessions,
-            'pageviews': total_pageviews,
-            'bounce_rate': bounce_rate,
-        },
+        'kpi': kpi,
         'chart_data': {
             'labels': chart_labels,
             'sessions': chart_sessions,