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/model/settings/settings.model.js

@@ -3,6 +3,7 @@ const brand = require('../../config/brand')
 const { parseJsonSetting } = require('../../utils/settingsJson')
 const { resolveThemeTokens } = require('../../utils/themeResolve')
 const { resolveBrandAssets } = require('../../utils/brandAssets')
+const { isDemo } = require('../../demo/mode')
 
 // Keys safe to expose on the public site.
 const PUBLIC_KEYS = [
@@ -215,9 +216,30 @@ async function getPublic() {
   // before doing so (the server would otherwise reject an unallowlisted one). The
   // custom-scheme callback works regardless of this flag.
   out.mobileAppLinks = String(all[MOBILE_APP_LINKS_KEY]) === 'true'
+  // The public demo's banner and sign-in panel (DEMO_MODE.md §4, §6). Present
+  // only with DEMO_MODE on, so a real shard's payload is unchanged. Written by
+  // `seed:demo` at every wipe; the demo lock keeps `demo_*` out of PUT
+  // /admin/settings, so a visitor cannot rewrite the published passwords.
+  if (isDemo()) out.demo = demoBlock(all[DEMO_STATE_KEY])
   return out
 }
 
+// `demo_state` is one JSON object: `{ resetsAt, logins: [{ username, password,
+// role }] }`. The passwords are public by design (§4) — the logins are printed
+// on the sign-in page and on runicgateway.com/beta. Read forgivingly: a missing
+// or broken row is a banner with no time and no logins, never a 500.
+const DEMO_STATE_KEY = 'demo_state'
+function demoBlock(raw) {
+  const state = parseJsonSetting(raw) || {}
+  const resetsAt = typeof state.resetsAt === 'string' && !Number.isNaN(Date.parse(state.resetsAt))
+    ? state.resetsAt
+    : null
+  const logins = (Array.isArray(state.logins) ? state.logins : [])
+    .filter((l) => l && typeof l.username === 'string' && typeof l.password === 'string')
+    .map((l) => ({ username: l.username, password: l.password, role: typeof l.role === 'string' ? l.role : null }))
+  return { resetsAt, logins }
+}
+
 /**
  * What the HTML shell needs, resolved exactly as getPublic() resolves it: the
  * effective favicon and logo, plus the theme token map for the boot <style>
@@ -285,4 +307,5 @@ module.exports = {
   registrationFlags,
   MOBILE_APP_LINKS_KEY,
   isMobileAppLinksEnabled,
+  DEMO_STATE_KEY,
 }