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

IMPLEMENTATION_PLAN.md

@@ -179,7 +179,7 @@ Drawer selection → update_drug_selection → app-state store → load_pathway_
 - **Checkpoint**: Changing date filter updates chart-data store with new pathway nodes ✓
 
 ### 3.3 KPI update callback
-- [ ] Create `dash_app/callbacks/kpi.py`:
+- [x] Create `dash_app/callbacks/kpi.py`:
   - `update_kpis` callback: Input=`chart-data` store, Output=KPI card values (4 outputs)
   - Extracts `unique_patients`, `total_drugs`, `total_cost` from chart-data
   - Formats numbers: patients with commas, cost as "£XXX.XM", drugs as plain number

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)

dash_app/callbacks/kpi.py

@@ -0,0 +1,41 @@
+"""Callback for updating KPI card values from chart-data store."""
+from dash import Input, Output, no_update
+
+
+def _format_cost(cost):
+    """Format cost as £X.XM or £X.XK."""
+    if cost >= 1_000_000:
+        return f"\u00a3{cost / 1_000_000:.1f}M"
+    if cost >= 1_000:
+        return f"\u00a3{cost / 1_000:.1f}K"
+    return f"\u00a3{cost:.0f}"
+
+
+def register_kpi_callbacks(app):
+    """Register KPI update callback."""
+
+    @app.callback(
+        Output("kpi-patients", "children"),
+        Output("kpi-drugs", "children"),
+        Output("kpi-cost", "children"),
+        Output("kpi-match", "children"),
+        Input("chart-data", "data"),
+        Input("app-state", "data"),
+    )
+    def update_kpis(chart_data, app_state):
+        """Update KPI card values when chart-data changes."""
+        if not chart_data:
+            return no_update, no_update, no_update, no_update
+
+        patients = chart_data.get("unique_patients", 0)
+        drugs = chart_data.get("total_drugs", 0)
+        cost = chart_data.get("total_cost", 0.0)
+
+        patients_str = f"{patients:,}" if patients else "\u2014"
+        drugs_str = str(drugs) if drugs else "\u2014"
+        cost_str = _format_cost(cost) if cost else "\u2014"
+
+        chart_type = (app_state or {}).get("chart_type", "directory")
+        match_str = "~93%" if chart_type == "indication" else "\u2014"
+
+        return patients_str, drugs_str, cost_str, match_str