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/internal/mail/mail.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"net/smtp"
 	"strings"
+	"time"
 )
 
 type Config struct {
@@ -53,3 +54,22 @@ func SendLicense(to, instanceName, blob string) error {
 			"Paste it into Settings → Licence on your Vantage install:\n\n%s\n",
 			instanceName, blob))
 }
+
+// SendInstanceReady tells a customer their cloud instance exists, where it is,
+// and when its licence runs out.
+//
+// The expiry is stated here rather than only in a later reminder: a Free licence
+// that quietly expires in a month is a surprise, and the first email is the one
+// people keep.
+func SendInstanceReady(to, instanceName, loginURL string, expires time.Time) error {
+	body := fmt.Sprintf("%s is ready.\n\n", instanceName)
+	if loginURL != "" {
+		body += "Sign in here:\n\n" + loginURL + "\n\n"
+	}
+	body += fmt.Sprintf(
+		"Your Free licence runs until %s. We will email you before then so you can renew it in one click.\n\n"+
+			"Sign in with the same email address and password you use for your Vantage account. "+
+			"Changing one does not change the other.\n",
+		expires.Format("2 January 2006"))
+	return send(to, instanceName+" is ready", body)
+}