fix: correct PnL math and add health probe

app/api/health/route.ts

@@ -0,0 +1,16 @@
+import { NextResponse } from 'next/server'
+
+/**
+ * Health check endpoint for Docker HEALTHCHECK
+ * Returns 200 OK if the server is running
+ */
+export async function GET() {
+  return NextResponse.json(
+    { 
+      status: 'healthy',
+      timestamp: new Date().toISOString(),
+      uptime: process.uptime(),
+    },
+    { status: 200 }
+  )
+}