docs(blog): how to publish, and why the app has two route groups

PUBLISHING.md carries the house style with the posts, so the standard
survives without the design doc to hand — including the rule that a post
states what a metric does not show, which is the strongest signal a
human wrote it.

CLAUDE.md gains the two constraints that are invisible from the code and
expensive to rediscover: metadata file conventions break if moved into a
route group, and the build must keep succeeding with DATABASE_URL unset.

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

claude.md

@@ -23,6 +23,46 @@ Key files:
 - `backend/data_loader.py` - Data queries, geocoding, legacy DataFrame compatibility
 - `backend/schemas.py` - Column mappings, metric definitions, LA code mappings
 
+### Content / CMS (Payload)
+
+Payload CMS runs **inside** the Next.js app — one image, one container, no
+separate service. It powers `/blog`; `/about` is a plain coded page.
+
+- **Admin panel:** `/admin`. The only authenticated surface on the site.
+  `noindex` via both `robots.txt` and `X-Robots-Tag`.
+- **CMS API:** `/cms-api`, **not** `/api`. `/api/*` is a catch-all proxy to
+  FastAPI (`app/(frontend)/api/[...path]`) which would silently swallow every
+  admin call and forward it to the backend. Mount points are defined once in
+  `lib/payloadRoutes.ts`.
+- **Database:** the existing Postgres, in its own `payload` schema, so no
+  pipeline operation on `public` — including
+  `scripts/migrate_csv_to_db.py --drop` — can reach blog content.
+- **Uploads:** the `payload_media` Docker volume at `/app/media`. Not
+  reproducible from the pipeline; must be backed up.
+- **New env vars:** `DATABASE_URL` and `PAYLOAD_SECRET` on the frontend service.
+  Staging must use a different `PAYLOAD_SECRET` from production.
+- Publishing workflow and house style: `nextjs-app/docs/PUBLISHING.md`.
+
+### Two route groups
+
+`nextjs-app/app/` has no root `layout.tsx`. It cannot: Payload's admin panel
+ships its own root layout rendering `<html>`/`<body>`, and Next permits
+multiple root layouts only when no `app/layout.tsx` exists.
+
+- `app/(frontend)/` — the site. Its `layout.tsx` is the site's root layout.
+- `app/(payload)/` — the admin panel and `/cms-api`.
+
+Route groups are invisible to routing, so every public URL is unchanged.
+
+**The metadata file conventions stay at the `app/` root** — `robots.ts`,
+`opengraph-image.tsx`, `icon.png`, `apple-icon.png`. Inside a route group Next
+treats them as segment-scoped: it renames `/icon.png` to `/icon-<hash>.png` and
+drops `/robots.txt` entirely. Route handlers are unaffected.
+
+The build must succeed with `DATABASE_URL` unset, because CI builds it that
+way. Never call `getCachedPayload()` at module scope, and never add
+`generateStaticParams` to a DB-backed route.
+
 ### Frontend (Vanilla JS)
 - Single-page application with hash-based routing
 - Chart.js for data visualization