feat(cms): install Payload and serve the admin panel

Payload 3.88 runs inside the Next app against the existing Postgres, in
its own 'payload' schema so no pipeline operation on public — the app
tables, Airflow's metadata, migrate_csv_to_db.py --drop — can reach blog
content.

Its REST API is mounted at /cms-api. /api is the FastAPI proxy's
catch-all, which would swallow every admin call and forward it to the
backend with no error. The mount points live in lib/payloadRoutes.ts so
there is one definition and a test can assert it without importing
Payload: it is ESM-only, next/jest will not transform it, and appending
transformIgnorePatterns cannot un-ignore a package. Forcing it through
transpilePackages would change how the production build bundles Payload
to serve a test, so the live proof that /api still reaches FastAPI stays
where it belongs — the e2e journeys, which call /api/schools.

The package becomes ESM ("type": "module"), which Payload's CLI requires:
richtext-lexical has top-level await and the config cannot be require()d.
Only two files needed renaming, jest.config.cjs and a build script.

The build is verified to succeed with DATABASE_URL and PAYLOAD_SECRET
both unset, which is how CI builds it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017YmbBhr8s7GusjDE12hrZM

nextjs-app/app/(payload)/admin/[[...segments]]/not-found.tsx

@@ -0,0 +1,16 @@
+import type { Metadata } from 'next';
+import config from '@payload-config';
+import { NotFoundPage, generatePageMetadata } from '@payloadcms/next/views';
+import { importMap } from '../importMap.js';
+
+type Args = {
+  params: Promise<{ segments: string[] }>;
+  searchParams: Promise<{ [key: string]: string | string[] }>;
+};
+
+export const generateMetadata = ({ params, searchParams }: Args): Promise<Metadata> =>
+  generatePageMetadata({ config, params, searchParams });
+
+export default function NotFound({ params, searchParams }: Args) {
+  return NotFoundPage({ config, importMap, params, searchParams });
+}

nextjs-app/app/(payload)/admin/[[...segments]]/page.tsx

@@ -0,0 +1,16 @@
+import type { Metadata } from 'next';
+import config from '@payload-config';
+import { RootPage, generatePageMetadata } from '@payloadcms/next/views';
+import { importMap } from '../importMap.js';
+
+type Args = {
+  params: Promise<{ segments: string[] }>;
+  searchParams: Promise<{ [key: string]: string | string[] }>;
+};
+
+export const generateMetadata = ({ params, searchParams }: Args): Promise<Metadata> =>
+  generatePageMetadata({ config, params, searchParams });
+
+export default function Page({ params, searchParams }: Args) {
+  return RootPage({ config, importMap, params, searchParams });
+}

nextjs-app/app/(payload)/admin/importMap.js

@@ -0,0 +1,6 @@
+import { CollectionCards as CollectionCards_f9c02e79a4aed9a3924487c0cd4cafb1 } from '@payloadcms/next/rsc'
+
+/** @type import('payload').ImportMap */
+export const importMap = {
+  "@payloadcms/next/rsc#CollectionCards": CollectionCards_f9c02e79a4aed9a3924487c0cd4cafb1
+}

nextjs-app/app/(payload)/cms-api/[...slug]/route.ts

@@ -0,0 +1,20 @@
+/*
+ * Payload's REST API, mounted at /cms-api rather than /api.
+ * See lib/payloadRoutes.ts — /api is the FastAPI proxy's catch-all.
+ */
+import config from '@payload-config';
+import {
+  REST_DELETE,
+  REST_GET,
+  REST_OPTIONS,
+  REST_PATCH,
+  REST_POST,
+  REST_PUT,
+} from '@payloadcms/next/routes';
+
+export const GET = REST_GET(config);
+export const POST = REST_POST(config);
+export const DELETE = REST_DELETE(config);
+export const PATCH = REST_PATCH(config);
+export const PUT = REST_PUT(config);
+export const OPTIONS = REST_OPTIONS(config);

nextjs-app/app/(payload)/cms-api/graphql-playground/route.ts

@@ -0,0 +1,4 @@
+import config from '@payload-config';
+import { GRAPHQL_PLAYGROUND_GET } from '@payloadcms/next/routes';
+
+export const GET = GRAPHQL_PLAYGROUND_GET(config);

nextjs-app/app/(payload)/cms-api/graphql/route.ts

@@ -0,0 +1,5 @@
+import config from '@payload-config';
+import { GRAPHQL_POST, REST_OPTIONS } from '@payloadcms/next/routes';
+
+export const POST = GRAPHQL_POST(config);
+export const OPTIONS = REST_OPTIONS(config);

nextjs-app/app/(payload)/layout.tsx

@@ -0,0 +1,27 @@
+/**
+ * Root layout for the Payload admin panel.
+ *
+ * This is a SECOND root layout: it renders its own <html>/<body>, as does
+ * app/(frontend)/layout.tsx. Next permits that only while no app/layout.tsx
+ * exists — which is why the site's routes were moved into (frontend). Adding
+ * an app/layout.tsx would nest the admin panel inside the site's nav, footer
+ * and providers and emit nested <html>.
+ */
+import type { ServerFunctionClient } from 'payload';
+import config from '@payload-config';
+import { RootLayout, handleServerFunctions } from '@payloadcms/next/layouts';
+import { importMap } from './admin/importMap.js';
+import '@payloadcms/next/css';
+
+const serverFunction: ServerFunctionClient = async function (args) {
+  'use server';
+  return handleServerFunctions({ ...args, config, importMap });
+};
+
+export default function PayloadLayout({ children }: { children: React.ReactNode }) {
+  return (
+    <RootLayout config={config} importMap={importMap} serverFunction={serverFunction}>
+      {children}
+    </RootLayout>
+  );
+}