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/lib/regime.ts

@@ -0,0 +1,44 @@
+import type { MarketRegime } from './types';
+
+export function regimeColor(label: MarketRegime['label']): string {
+  switch (label) {
+    case 'bullish':
+      return 'text-emerald-400';
+    case 'bearish':
+      return 'text-red-400';
+    case 'neutral':
+      return 'text-amber-400';
+    default:
+      return 'text-gray-400';
+  }
+}
+
+export function regimeDot(label: MarketRegime['label']): string {
+  switch (label) {
+    case 'bullish':
+      return 'bg-emerald-400';
+    case 'bearish':
+      return 'bg-red-400';
+    case 'neutral':
+      return 'bg-amber-400';
+    default:
+      return 'bg-gray-600';
+  }
+}
+
+export function regimeHeadline(r: MarketRegime): string {
+  const b = r.benchmark ?? 'SPY';
+  if (r.label === 'unknown') return `${b} trend unknown`;
+  const pct =
+    r.pct_above_200 != null
+      ? ` · ${r.pct_above_200 >= 0 ? '+' : ''}${r.pct_above_200.toFixed(1)}% vs 200-day`
+      : '';
+  return `${b} ${r.label}${pct}`;
+}
+
+/** Whether a setup direction fights the prevailing market regime. */
+export function isCounterTrend(direction: string, label: MarketRegime['label']): boolean {
+  if (label === 'bullish') return direction === 'short';
+  if (label === 'bearish') return direction === 'long';
+  return false;
+}