fix: rename next.config.ts → next.config.mjs for Next.js 14 compatibility

next.config.ts is only supported in Next.js 15+. Next.js 14 requires
.js or .mjs. Also switched serverExternalPackages to the Next.js 14
experimental.serverComponentsExternalPackages key.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

next.config.mjs

@@ -0,0 +1,9 @@
+/** @type {import('next').NextConfig} */
+const nextConfig = {
+  output: "standalone",
+  experimental: {
+    serverComponentsExternalPackages: ["pg", "ioredis"],
+  },
+};
+
+export default nextConfig;