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>

app/routers/market.py

@@ -7,7 +7,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from app.dependencies import get_db, require_access, require_admin
 from app.models.user import User
 from app.schemas.common import APIEnvelope
-from app.services import regime_monitor_service
+from app.services import event_study_service, regime_monitor_service
 from app.services.backtest_service import get_backtest_report
 from app.services.market_regime_service import get_market_regime
 
@@ -117,3 +117,15 @@ async def refresh_regime_fundamentals(
     """Ask the configured LLM to re-estimate F1/F3 now (forces past a lock)."""
     data = await regime_monitor_service.refresh_fundamental_overrides(db, force=True)
     return APIEnvelope(status="success", data=data)
+
+
+@router.get("/regime/event-study", response_model=APIEnvelope)
+async def regime_event_study(
+    _user: User = Depends(require_access),
+    db: AsyncSession = Depends(get_db),
+) -> APIEnvelope:
+    """Cached early-warning event study (lead time vs. historical drawdowns).
+
+    None until the manual "Event Study" job has run (Admin → Jobs)."""
+    data = await event_study_service.get_event_study_report(db)
+    return APIEnvelope(status="success", data=data)