feat: add KPI update callback with formatted patient/drug/cost display (Task 3.3)

dash_app/callbacks/__init__.py

@@ -5,6 +5,8 @@ def register_callbacks(app):
     """Register all Dash callbacks with the app instance."""
     from dash_app.callbacks.filters import register_filter_callbacks
     from dash_app.callbacks.chart import register_chart_callbacks
+    from dash_app.callbacks.kpi import register_kpi_callbacks
 
     register_filter_callbacks(app)
     register_chart_callbacks(app)
+    register_kpi_callbacks(app)