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>

Dockerfile

@@ -21,7 +21,7 @@ COPY . .
 RUN pnpm prisma generate
 
 ENV NEXT_TELEMETRY_DISABLED=1
-ENV NODE_OPTIONS="--max-old-space-size=1536"
+ENV NODE_OPTIONS="--max-old-space-size=2048"
 RUN pnpm build
 
 # ── runner ──────────────────────────────────────────────────────────────────