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/test/moduleLoader.test.js

@@ -550,8 +550,10 @@ test('ctx exposes exactly the documented surface, and is frozen', () => {
   // API 1.17.0 added `identities` (STEAM_SIGNIN.md §4): READ only, one provider
   // at a time. Linking stays core's, and a module takes part through the two
   // hooks on `api`, never by writing an identity itself.
+  // API 1.18.0 added `demoMode` (DEMO_MODE.md §3.4): a boolean, for the locks
+  // the module enforces in its own handlers.
   assert.deepEqual(probe.keys, [
-    'activity', 'auth', 'db', 'events', 'express', 'identities', 'inbox', 'log', 'middleware', 'moduleId', 'paths',
+    'activity', 'auth', 'db', 'demoMode', 'events', 'express', 'identities', 'inbox', 'log', 'middleware', 'moduleId', 'paths',
     'posts', 'push', 'secretBox', 'settings', 'site', 'teams', 'uploads', 'users', 'validator',
   ])
   //  is core's limiter FACTORY, not a limiter: a module states its own