feat(admin): POST /api/instances creates a Free cloud instance

Provisions the control-plane instance and its owner, records the
admin_instances row, issues and injects a Free licence, and emails the
customer where it is and when it expires.

Licence issuance and email cannot fail the request. The instance exists
and the customer can sign in; rolling back something they can already see
would be worse than shipping it unlicensed for staff to fix.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

admin/cmd/main.go

@@ -30,6 +30,7 @@ func main() {
 	}
 
 	licensing.SetSigningKey(cfg.SigningKey)
+	api.SetAppLoginURL(cfg.AppLoginURL)
 
 	mail.Init(mail.Config{
 		Host: cfg.SMTPHost, Port: cfg.SMTPPort, From: cfg.SMTPFrom,