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.