fix(web): payload at /payload route (cuidecar pattern)

Replicate working cuidecar Payload setup:
- require() instead of ESM import for withPayload
- routes.admin = "/payload" to avoid /admin conflicts
- (payload)/payload/ route group with own layout + importMap

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/payload.config.ts

@@ -15,6 +15,10 @@ const usePostgres = !!process.env.DATABASE_URL;
 export default buildConfig({
   secret: process.env.PAYLOAD_SECRET || "claudemesh-dev-secret-change-in-production",
 
+  routes: {
+    admin: "/payload",
+  },
+
   admin: {
     user: "users",
     meta: {