Sign-in is set up from the console, and a directory tried before it is kept

People → Sign-in shows single sign-on, Active Directory and SCIM, and
the directory's last sync. E sets up Entra ID from the tenant and client
IDs; D sets up Active Directory, tried from the control plane first:
each server bound, people who may sign in, members of each mapped group.
The secrets go into the store as store:auth/..., sealed by the master
key, and the CA into the settings, so no control plane needs a file.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

internal/control/adminapi/api.go

@@ -76,6 +76,7 @@ type API struct {
 	// train is adapter training; nil without a store and agents.
 	train    *TrainAdmin
 	settings *Settings
+	auth     *AuthAdmin
 	// restartAgents tells every connected agent to start again.
 	restartAgents func(reason string) []string
 	// revoke records a removed node, so its certificate is refused.
@@ -129,6 +130,7 @@ func (a *API) Register(mux Mux) {
 	a.registerTrain(mux)
 	a.registerGuard(mux)
 	a.registerSettings(mux)
+	a.registerAuth(mux)
 	mux.HandleFunc("POST /api/v1/agents/restart", a.restartAgentsHandler)
 	mux.HandleFunc("GET /api/v1/audit", a.auditEvents)
 	mux.HandleFunc("GET /api/v1/usage/export", a.usageExport)