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 });
+}