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/services/regime_monitor_service.py

@@ -681,6 +681,30 @@ async def get_regime_monitor(db: AsyncSession) -> dict:
     return result
 
 
+async def get_regime_history(db: AsyncSession, days: int = 400) -> list[dict]:
+    """Daily history of the index, early-warning, and combined scores for the
+    score-over-time chart. One point per snapshot date, ascending."""
+    cutoff = date.today() - timedelta(days=days)
+    res = await db.execute(
+        select(RegimeSnapshot)
+        .where(RegimeSnapshot.date >= cutoff)
+        .order_by(RegimeSnapshot.date.asc())
+    )
+    out: list[dict] = []
+    for row in res.scalars().all():
+        try:
+            data = json.loads(row.breakdown_json)
+        except (TypeError, ValueError):
+            data = {}
+        out.append({
+            "date": row.date.isoformat(),
+            "index": row.total_score,
+            "early_warning": (data.get("early_warning") or {}).get("score"),
+            "combined": (data.get("combined") or {}).get("score"),
+        })
+    return out
+
+
 # ---------------------------------------------------------------------------
 # F1/F3 via grounded LLM (reuses the configured sentiment provider)
 # ---------------------------------------------------------------------------