feat: add challenge and red-blue competitions across API and web

web/app/red-blue-challenges/page.tsx

@@ -0,0 +1,23 @@
+import Link from 'next/link'
+import { fetchRedBlueChallenges } from '@/service/redBlueChallenges'
+
+export default async function RedBlueChallengesPage() {
+  const items = await fetchRedBlueChallenges()
+  return (
+    <div className="px-6 py-8">
+      <h1 className="mb-4 text-xl font-semibold">Red / Blue Challenges</h1>
+      <ul className="space-y-2">
+        {items.map(i => (
+          <li key={i.id} className="rounded border p-3">
+            <Link href={`/red-blue-challenges/${i.id}`} className="text-primary hover:underline">
+              {i.name}
+            </Link>
+            {i.description && (
+              <p className="mt-1 text-sm text-gray-500">{i.description}</p>
+            )}
+          </li>
+        ))}
+      </ul>
+    </div>
+  )
+}