feat: risk tier selector on Trading page + confidence filter on Recommendations

- Trading page: added conservative/moderate/aggressive selector that
  updates the trading engine config via PUT /api/trading/config
- Recommendations page: added risk tier dropdown that defaults to the
  engine's current tier and filters recs by the tier's min_confidence
- Backend: added min_confidence query param to GET /api/recommendations
- Risk tier thresholds: conservative ≥0.75, moderate ≥0.55, aggressive ≥0.40

frontend/src/pages/Trading.tsx

@@ -10,7 +10,7 @@ import {
   useCompetitiveStatus,
   useToggleCompetitive,
 } from '../api/hooks';
-import { useResetPaperTrading } from '../api/tradingHooks';
+import { useResetPaperTrading, useTradingStatus, useUpdateTradingConfig } from '../api/tradingHooks';
 import { StatusBadge, LoadingSpinner, Card } from '../components/ui';
 
 export function TradingPage() {
@@ -21,6 +21,8 @@ export function TradingPage() {
   const { data: competitiveStatus } = useCompetitiveStatus();
   const setMode = useSetTradingMode();
   const resetTrading = useResetPaperTrading();
+  const { data: tradingStatus } = useTradingStatus();
+  const updateConfig = useUpdateTradingConfig();
   const reviewApproval = useReviewApproval();
   const toggleMacro = useToggleMacro();
   const toggleCompetitive = useToggleCompetitive();
@@ -95,6 +97,41 @@ export function TradingPage() {
         isResetting={resetTrading.isPending}
       />
 
+      {/* Risk Tier */}
+      <Card>
+        <h2 className="mb-3 text-sm font-medium text-gray-400">Risk Tier</h2>
+        <p className="mb-3 text-[10px] text-gray-600">
+          Controls confidence gates, position sizing, and portfolio heat limits for the trading engine.
+        </p>
+        <div className="flex items-center gap-3">
+          {(['conservative', 'moderate', 'aggressive'] as const).map((tier) => {
+            const currentTier = tradingStatus?.risk_tier ?? 'moderate';
+            const descriptions: Record<string, string> = {
+              conservative: 'Min confidence 0.75, max 5% position, 10% heat',
+              moderate: 'Min confidence 0.55, max 10% position, 20% heat',
+              aggressive: 'Min confidence 0.40, max 15% position, 30% heat',
+            };
+            return (
+              <button
+                key={tier}
+                onClick={() => {
+                  if (tier !== currentTier) updateConfig.mutate({ risk_tier: tier });
+                }}
+                className={`rounded-md px-4 py-2 text-sm font-medium capitalize transition-colors ${
+                  currentTier === tier
+                    ? 'bg-brand-600 text-white'
+                    : 'border border-surface-700 bg-surface-900 text-gray-400 hover:bg-surface-800'
+                }`}
+                aria-pressed={currentTier === tier}
+                title={descriptions[tier]}
+              >
+                {tier}
+              </button>
+            );
+          })}
+        </div>
+      </Card>
+
       {/* Macro Signal Layer Toggle */}
       <Card>
         <h2 className="mb-3 text-sm font-medium text-gray-400">Macro Signal Layer</h2>