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>

app/routers/market.py

@@ -1,6 +1,6 @@
 """Market-level endpoints (benchmark regime + AI/Tech regime-change monitor)."""
 
-from fastapi import APIRouter, Depends
+from fastapi import APIRouter, Depends, Query
 from pydantic import BaseModel
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -129,3 +129,14 @@ async def regime_event_study(
     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)
+
+
+@router.get("/regime/history", response_model=APIEnvelope)
+async def regime_history(
+    days: int = Query(default=400, ge=7, le=2000),
+    _user: User = Depends(require_access),
+    db: AsyncSession = Depends(get_db),
+) -> APIEnvelope:
+    """Daily history of the index / early-warning / combined scores (for the chart)."""
+    data = await regime_monitor_service.get_regime_history(db, days=days)
+    return APIEnvelope(status="success", data=data)