feat(multi-site): live catalog replication + identical-directory signing key

The shipped join flow (v2.2.0-v2.3.0) was a one-time snapshot: a spoke's
catalog never updated after joining. This adds the two pieces that were
explicitly designed but missing:

- Live replication: a spoke registers its own endpoint with the master
  right after joining (POST /api/site/spokes, Bearer join-key), receiving
  a pushToken. Every successful catalog write on the master now fires a
  fire-and-forget resync ping (utils/site_replicate.js) at every known
  spoke, concurrently -- one unreachable spoke never blocks or delays
  another (wired into the existing write-gate middleware in
  api_directory_admin.js). The spoke's POST /api/site/resync handler
  reuses the already-tested export+import path rather than applying a
  partial diff.

- Identical directories: POST /api/site/export now best-effort includes
  the master's agent-signing key; a spoke adopts it via agent_keys.adopt()
  on both join and every resync, so every site's sso-manager can validly
  sign a command for any agent enrolled anywhere -- the accepted tradeoff
  discussed for this deployment's scale (blast radius for simplicity).

New SiteSpoke model tracks registered spokes (endpoint + pushToken);
registered it in models/index.js (a real bug the e2e test below caught --
SiteSpoke.list() 500'd with "Cannot read properties of null (reading
'adapter')" until the model was added to initORM's model list).

Verified end-to-end against docker-compose.multisite-e2e.yml: mint join
key -> join with selfUrl -> write a NEW resource on master post-join ->
poll the spoke -> it shows up within a few seconds via the resync push,
no manual re-join needed. MULTISITE E2E PASS.

Unit tests: nodejs/tests/site_replicate.test.js (concurrent fan-out, one
failing spoke doesn't block another, empty-registry and list()-throws
edge cases).

nodejs/models/index.js

@@ -22,6 +22,7 @@ const { SharedSecretGrant } = require('./shared_secret_grant');
 const { VaultAppToken } = require('./vault_app_token');
 const { Agent, AgentJoinKey } = require('./agent');
 const { SiteJoinKey } = require('./site_join_key');
+const { SiteSpoke } = require('./site_spoke');
 async function initORM() {
   const ormConf = conf.orm || {
     dialect: 'sqlite',
@@ -36,7 +37,7 @@ async function initORM() {
       conf: { orm: ormConf },
       models: [
         Resource, ResourceEdge, ResourceGroup, AccessRequest, Webhook, PluginInstance,
-        SharedSecret, SharedSecretGrant, VaultAppToken, Agent, AgentJoinKey, SiteJoinKey,
+        SharedSecret, SharedSecretGrant, VaultAppToken, Agent, AgentJoinKey, SiteJoinKey, SiteSpoke,
         Token, AuthToken, InviteToken, ImpersonationToken, PasswordResetToken, OtpToken, ServiceToken
       ]
     });