feat: breadth-divergence early-warning indicator + event study

Adds a leading-by-construction candidate and the harness to measure whether it
actually leads regime breaks, before any of it earns weight in the live index.

- breadth_service: % of the stored universe above its own 200-DMA + a divergence
  score (benchmark price up while breadth falls, nudged by low breadth). Genuinely
  leading because it keys on divergence, not level. Not wired into the live score.
- event_study_service: detect drawdown events on the benchmark, then measure each
  indicator's median lead time (event-centered) and precision/recall vs. the base
  rate (signal-centered). Compares breadth-divergence against the deterministic
  coincident price composite (reuses the regime price sub-scores). Price/breadth
  only — reproducible, no LLM/FRED.
- Manual "Event Study" job (Admin → Jobs), GET /regime/event-study, and an
  inline early-warning panel on the Regime tab with an honest small-sample caveat.

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

frontend/src/api/regime.ts

@@ -1,10 +1,19 @@
 import apiClient from './client';
-import type { RegimeMonitor, RegimeConfig, RegimeFundamentals } from '../lib/types';
+import type {
+  RegimeMonitor,
+  RegimeConfig,
+  RegimeFundamentals,
+  EventStudyReport,
+} from '../lib/types';
 
 export function getRegimeMonitor() {
   return apiClient.get<RegimeMonitor>('regime/monitor').then((r) => r.data);
 }
 
+export function getEventStudy() {
+  return apiClient.get<EventStudyReport | null>('regime/event-study').then((r) => r.data);
+}
+
 export function getRegimeConfig() {
   return apiClient.get<RegimeConfig>('regime/config').then((r) => r.data);
 }