fix(web): restore serverExternalPackages for Payload + use --webpack for build

Root cause: Next.js 16 defaults to Turbopack for builds, but Payload CMS's
richtext-lexical imports .css files that fail during route collection in
Node ESM context.

Fix: add @payloadcms/richtext-lexical and @payloadcms/next back to
serverExternalPackages so Next.js skips their internal imports during
route collection. Use --webpack explicitly since Turbopack production
builds are incompatible with Payload (payloadcms/payload#14786).

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

apps/web/next.config.ts

@@ -87,6 +87,8 @@ const config: NextConfig = {
     "payload",
     "@payloadcms/db-postgres",
     "@payloadcms/db-sqlite",
+    "@payloadcms/richtext-lexical",
+    "@payloadcms/next",
     "sharp",
   ],
   turbopack: {

apps/web/package.json

@@ -4,7 +4,7 @@
   "private": true,
   "type": "module",
   "scripts": {
-    "build": "next build",
+    "build": "next build --webpack",
     "clean": "git clean -xdf .cache .next .turbo node_modules",
     "dev": "next dev",
     "format": "prettier --check . --ignore-path ../../.gitignore",