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/lib/types.ts

@@ -311,6 +311,38 @@ export interface RegimeConfig {
   fundamental_staleness_days: number;
 }
 
+// Event study — measured lead time of early-warning indicators vs. drawdowns
+export interface EventStudyLeadStats {
+  median_lead_days: number | null;
+  events_with_signal: number;
+  events_total: number;
+  mean_path: { rel_day: number; value: number }[];
+  signal: {
+    base_rate: number;
+    horizon_days: number;
+    rows: { threshold: number; precision: number | null; recall: number | null; alarms: number }[];
+  };
+}
+
+export interface EventStudyReport {
+  available: boolean;
+  reason?: string;
+  generated_at?: string;
+  params?: {
+    benchmark: string;
+    event_threshold_pct: number;
+    horizon_days: number;
+    warn_threshold: number;
+  };
+  events?: { date: string; index: number; depth_pct: number }[];
+  indicators?: {
+    breadth_divergence: EventStudyLeadStats;
+    coincident_price: EventStudyLeadStats;
+  };
+  lead_delta_days?: number | null;
+  recent_breadth?: { date: string; breadth: number; divergence: number | null }[];
+}
+
 export interface AlertConfig {
   enabled: boolean;
   telegram_chat_id: string;