feat(demo): DEMO_MODE lock, seed:demo and the demo banner (MODULE_API 1.18.0)

Core half of the public demo (docs/website/DEMO_MODE.md §10 step 1, website#222).
Nothing here runs unless DEMO_MODE=true.

- server/src/demo/: the lock at /api/v1, ahead of every router. It covers every
  Tier A row of §3.3, the shared-login conditions, POST /auth/register (DM16),
  DELETE /auth/me/sessions/:id on a demo account (DM17), and the PUT
  /admin/settings keys that reach a Tier A lock. Any multipart request is refused
  app-wide. noindex and robots.txt. A condition that cannot decide refuses.
- Loader: demoLocks in module.json (writes only, under the module's own mounts),
  ctx.demoMode, and api.onDemoSeed (DM15). MODULE_API 1.18.0 on both halves.
- GET /public/settings carries demo: { resetsAt, logins } on the demo only.
- npm run seed:demo -- <fixture>: upserts the four accounts, sets the site live
  with registration disabled, seeds content on the first run only, calls each
  module's onDemoSeed, and stamps demo_state. It resolves $env: strings and
  refuses to run off the demo.
- Client: the banner, one DEMO_LOCKED toast fed from the request primitive (so
  module screens get it too), and the published logins on both sign-in pages.
  Dashboard and register say the demo message instead of blaming a role.
- OpenAPI: DemoLocked schema, the demo block, and a note that any write may
  answer DEMO_LOCKED on the demo.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E14m6SuuY6i1vASFeGDBeY

server/src/app.js

@@ -21,6 +21,8 @@ const createLogger = require('./utils/logger')
 const htmlShell = require('./utils/htmlShell')
 const { applyTrustProxy, trustProxyDebug } = require('./utils/trustProxy')
 const botScore = require('./middleware/botScore')
+const { isDemo } = require('./demo/mode')
+const demoLock = require('./demo/locks')
 
 const httpLog = createLogger('http')
 const errLog = createLogger('error')
@@ -84,6 +86,23 @@ app.use(morgan(accessFormat, { stream: { write: (line) => httpLog.info(line.trim
 app.use(express.json({ limit: '2mb' }))
 app.use(cookieParser())
 
+// ── The public demo ───────────────────────────────────────────────────
+// DEMO_MODE=true turns this deployment into demo.runicgateway.com
+// (docs/website/DEMO_MODE.md). Unset, nothing here is mounted, so a real shard
+// runs none of it. After the body parser, because one lock reads the body of
+// PUT /admin/settings; after cookieParser, because the shared-login locks read
+// the session; ahead of `/api` and of the SPA's static files (robots.txt).
+// The module locks are asked per request: the loader runs further down.
+if (isDemo()) {
+  app.use(demoLock.noindex)
+  app.get('/robots.txt', (req, res) => {
+    // #swagger.ignore = true
+    demoLock.robots(req, res)
+  })
+  app.use(demoLock.refuseMultipart)
+  app.use('/api/v1', demoLock.createLock(() => modules.demoLocks()))
+}
+
 // ── Paths ─────────────────────────────────────────────────────────────
 const SERVER_ROOT = path.join(__dirname, '..')
 const REPO_ROOT = path.join(SERVER_ROOT, '..')