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/api/regime.ts

@@ -4,12 +4,19 @@ import type {
   RegimeConfig,
   RegimeFundamentals,
   EventStudyReport,
+  RegimeHistoryPoint,
 } from '../lib/types';
 
 export function getRegimeMonitor() {
   return apiClient.get<RegimeMonitor>('regime/monitor').then((r) => r.data);
 }
 
+export function getRegimeHistory(days = 400) {
+  return apiClient
+    .get<RegimeHistoryPoint[]>('regime/history', { params: { days } })
+    .then((r) => r.data);
+}
+
 export function getEventStudy() {
   return apiClient.get<EventStudyReport | null>('regime/event-study').then((r) => r.data);
 }