feat(multi-site): auto-assign LDAP ServerID + replication hosts at join time

OpenLDAP N-way multi-master replication (docs/replication.md) required
an operator to hand-set LDAP_SERVER_ID (unique per site) and
LDAP_REPLICATION_HOSTS (every OTHER site's LDAP URL, kept in sync by
hand across every node) -- real coordination work, and easy to get
wrong or let drift as sites are added.

Automates the coordination the master is already in a position to do:
- SiteSpoke gets ldapServerId, auto-assigned (next free from 2 upward,
  1 reserved for the master) at registration and reused across
  re-registrations -- same pattern as jump-host's mesh index.
- ldapHost is derived from each site's already-known HTTP(S) endpoint
  (same hostname, port 636) rather than a separately-configured field
  that could drift from it.
- New utils/ldap_replication.js (nextFreeLdapServerId, ldapHostFor),
  shared between the spoke-facing GET /api/site/ldap-peers (Bearer
  site join key, returns this caller's own ID + every peer) and the
  master-local GET /directory-admin/ldap-replication-config (computes
  its own config directly from SiteSpoke, no HTTP round-trip needed).

Verified against real running containers (docker-compose.multisite-e2e.yml):
after a real join, the master's computed config correctly includes the
spoke as a peer with an assigned ID, and the spoke's own fetched
config matches that ID and correctly excludes itself from its own
peer list.

Known limitation, documented in docs/replication.md: the master's own
LDAP_REPLICATION_HOSTS only gets recomputed when ITS setup.sh is
re-run (or an admin re-applies it directly) -- there's no live push to
an already-running master when a new spoke joins. A spoke's own config
is re-checked on every setup.sh run, which is the common/recurring
event; the master side is a documented manual step for now rather than
a live hot-reload (which would need OpenLDAP's dynamic cn=config
backend -- a bigger change, deliberately out of scope here to avoid
risking a live directory's LDAP replication on undertested config).

nodejs/models/site_spoke.js

@@ -39,7 +39,15 @@ class SiteSpoke extends Model {
 		noInbound: { type: 'boolean', default: false },
 		meshIp: { type: 'string' },
 		publicHost: { type: 'string' },
-		relayNote: { type: 'string' }
+		relayNote: { type: 'string' },
+		// OpenLDAP multi-master replication (docs/replication.md): a unique
+		// small integer this spoke's slapd.conf ServerID must use. Assigned
+		// once at registration (see api_site.js's nextFreeLdapServerId),
+		// reused on re-registration -- a spoke that re-registers after a
+		// restart must not get bumped to a new ID, same reasoning as
+		// jump-host's meshIndex. The master reserves 1 for itself, never
+		// assigned here.
+		ldapServerId: { type: 'integer' }
 	};
 
 	toPublic() {