feat: score-history chart on the regime tab

Plots the index, early-warning, and combined scores over time beneath the live
gauges, with a 1M/3M/6M/All range toggle and band reference lines — so the trend
and any divergence between the scores is visible, not just today's snapshot.

- Backend: GET /regime/history + get_regime_history (the three scores per
  snapshot date from regime_snapshots).
- Frontend: recharts line chart, lazy-loaded so recharts ships in its own
  regime-tab chunk instead of nearly doubling the main bundle.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/pages/RegimePage.tsx

@@ -1,4 +1,4 @@
-import { useState, type ReactNode } from 'react';
+import { useState, lazy, Suspense, type ReactNode } from 'react';
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
 import { PageHeader } from '../components/ui/PageHeader';
 import { Callout } from '../components/ui/Callout';
@@ -15,6 +15,9 @@ import {
   refreshRegimeFundamentals,
   getEventStudy,
 } from '../api/regime';
+
+// Lazy so recharts (heavy) ships in its own chunk, loaded only on this tab.
+const ScoreHistoryChart = lazy(() => import('../components/regime/ScoreHistoryChart'));
 import type {
   RegimeBand,
   RegimeSignal,
@@ -562,6 +565,9 @@ export default function RegimePage() {
               regime config.</>
             }
           />
+          <Suspense fallback={<SkeletonCard className="h-72" />}>
+            <ScoreHistoryChart />
+          </Suspense>
           {monitor.data.breakdown && <Breakdown breakdown={monitor.data.breakdown} />}
         </>
       )}