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/lib/types.ts

@@ -282,6 +282,13 @@ export interface RegimeSubScore {
   delta_30?: number | null;
 }
 
+export interface RegimeHistoryPoint {
+  date: string;
+  index: number;
+  early_warning: number | null;
+  combined: number | null;
+}
+
 export interface RegimeMonitor {
   available: boolean;
   reason?: string;