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/api/market.ts

@@ -0,0 +1,6 @@
+import apiClient from './client';
+import type { MarketRegime } from '../lib/types';
+
+export function getMarketRegime() {
+  return apiClient.get<MarketRegime>('market/regime').then((r) => r.data);
+}