feat(site): join UI, spoke read-only enforcement, live WAN health, fresh-install guard

Completes the multi-site join layer on top of the v2.2.0 endpoints:

- UI (Master Site modal): a fresh install (canJoin) gets a 'Join an Existing
  Site' form (master URL + stj_ key); a master gets a 'Site Join Keys' manager
  (mint/revoke/list, key shown once); WAN Sync Health now reflects a live probe.
- POST /api/site/ping (Bearer stj_ key, no admin session): lightweight master
  reachability probe for WAN health (cheap vs /export).
- Spoke read-only: directory-write routes (resources/edges/groups/secrets/
  grants/driver-action/discovered) reject with 403 pointing at the master.
- Fresh-install guard: /api/site/join refuses unless no users beyond the
  bootstrap admin and no enrolled agents (siteIsFresh), and site-status exposes
  canJoin so the UI only offers join on a genuinely fresh install. The
  bootstrap's seeded default resources are NOT the signal (they always exist).
- The spoke stores the join key (masterJoinKey) in /config/site.json so WAN
  health (and a future write-proxy) can reach the master.
- Tests: siteIsFresh cases in tests/site_join.test.js.

nodejs/tests/site_join.test.js

@@ -1,6 +1,6 @@
 'use strict';
 
-const { scalarResource, scalarEdge, importDirectory, ldapAddArgs, baseDnFrom } = require('../utils/site_join');
+const { scalarResource, scalarEdge, importDirectory, ldapAddArgs, baseDnFrom, siteIsFresh } = require('../utils/site_join');
 
 // In-memory model stubs so importDirectory can be exercised without a DB.
 function makeStore() {
@@ -119,3 +119,32 @@ test('baseDnFrom prefers stack.ldapBaseDn and falls back to the bind DN', () =>
   expect(baseDnFrom({ ldap: { bindDN: 'cn=admin,dc=example,dc=com' } })).toBe('dc=example,dc=com');
   expect(baseDnFrom({ ldap: { bindDN: 'cn=admin' } })).toBe('');
 });
+
+// The fresh-install guard: only no-users-beyond-admin + no-agents may join.
+test('siteIsFresh is true with only the bootstrap admin and no agents', async () => {
+  const User = { listDetail: async () => [{ uid: 'admin', isServiceAccount: false }] };
+  const Agent = { list: async () => [] };
+  expect(await siteIsFresh({ User, Agent })).toBe(true);
+});
+
+test('siteIsFresh is false with a second real user', async () => {
+  const User = { listDetail: async () => [{ uid: 'admin' }, { uid: 'bob' }] };
+  const Agent = { list: async () => [] };
+  expect(await siteIsFresh({ User, Agent })).toBe(false);
+});
+
+test('siteIsFresh is false with an enrolled agent', async () => {
+  const User = { listDetail: async () => [{ uid: 'admin' }] };
+  const Agent = { list: async () => [{ id: 'a1' }] };
+  expect(await siteIsFresh({ User, Agent })).toBe(false);
+});
+
+test('siteIsFresh ignores service accounts', async () => {
+  const User = { listDetail: async () => [
+    { uid: 'admin' },
+    { uid: 'sso-svc', isServiceAccount: true },
+    { uid: 'ldapclient', isServiceAccount: true }
+  ] };
+  const Agent = { list: async () => [] };
+  expect(await siteIsFresh({ User, Agent })).toBe(true);
+});