fix(energy): report real kWh in the cost Summary instead of mislabelled money

The Summary cards labelled `metered_import` / `metered_export` as "(kWh)", but
both fields are monetary totals (Σ import_cost / Σ export_revenue).  Today's
page therefore showed "Import 1.339 kWh" when the meter had actually imported
4.188 kWh — the 1.339 was EUR.  Cross-checked against the DSMR cumulative
registers and Home Assistant: our energy figures were correct all along, only
the label was wrong.

summarize() now also aggregates the metered energy, reusing the already-fetched
non-degraded rows so no extra query is issued:

  metered_import_kwh = Σ (d1_kwh + d2_kwh)
  metered_export_kwh = Σ (r1_kwh + r2_kwh)

The Import/Export cards show kWh as the headline figure and keep the monetary
equivalent as a sub-line, so the split between energy cost and standing
charges/credits behind total_payable stays visible.

The `_kwh` suffix is now the only thing separating energy from money in this
payload, so the docstrings on both summarize() and SummaryResponse call that
out explicitly.

app/integrations/expose.py reads only the money keys, so the HA outbound
sensors are unaffected by the additive fields.

frontend/src/energy/CostView.test.tsx

@@ -61,9 +61,13 @@ const COST_PERIOD = {
 
 const SUMMARY = {
   currency: 'EUR',
+  // Money totals and kWh totals are deliberately distinct so the assertions
+  // below prove the cards read the *_kwh fields, not the monetary ones.
   metered_import: 10.5,
   metered_export: 2.3,
   metered_net: 8.2,
+  metered_import_kwh: 33.3,
+  metered_export_kwh: 44.4,
   fixed_costs: 5.0,
   credits: 50.0,
   total_payable: 12.5,
@@ -145,9 +149,14 @@ describe('CostView', () => {
       expect(screen.getByTestId('summary-import')).toBeInTheDocument()
     })
 
-    expect(screen.getByTestId('summary-import')).toHaveTextContent('10.500')
-    expect(screen.getByTestId('summary-export')).toHaveTextContent('2.300')
+    // Main figure is energy (kWh), taken from the *_kwh fields.
+    expect(screen.getByTestId('summary-import')).toHaveTextContent('33.300')
+    expect(screen.getByTestId('summary-export')).toHaveTextContent('44.400')
     expect(screen.getByTestId('summary-total')).toHaveTextContent('12.50')
+
+    // Sub-line carries the monetary equivalent, so money is still visible.
+    expect(screen.getByTestId('summary-import-sub')).toHaveTextContent('10.50 EUR')
+    expect(screen.getByTestId('summary-export-sub')).toHaveTextContent('2.30 EUR')
   })
 
   it('shows recompute confirmation modal on button click', async () => {