add market-regime guard (SPY trend) — inform + warn

New market_regime_service computes a benchmark (SPY) trend from its 50/200-day
SMAs, cached in a SystemSetting and refreshed by a nightly job; GET /market/regime
exposes it. Dashboard shows a regime banner; setup cards flag a counter-trend
caution when a setup fights the regime (LONG in a bearish market / SHORT in a
bullish one). Informational only — nothing is suppressed.

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

frontend/src/hooks/useMarketRegime.ts

@@ -0,0 +1,10 @@
+import { useQuery } from '@tanstack/react-query';
+import * as marketApi from '../api/market';
+
+export function useMarketRegime() {
+  return useQuery({
+    queryKey: ['market-regime'],
+    queryFn: () => marketApi.getMarketRegime(),
+    staleTime: 60_000,
+  });
+}