fix(build): use webpack instead of Turbopack for production build

Turbopack (Next.js 16 default) OOM-kills on low-memory servers — BuildKit
loses the connection and reports EOF. Webpack uses significantly less RAM.
Also bumped NODE_OPTIONS heap limit to 2048MB.

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

package.json

@@ -5,7 +5,7 @@
   "packageManager": "pnpm@11.6.0",
   "scripts": {
     "dev": "next dev",
-    "build": "next build",
+    "build": "next build --webpack",
     "start": "next start",
     "lint": "eslint"
   },