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: {