feat(admin): renew a Free instance from the portal

Available from seven days before expiry and, deliberately, at any point
after it up to deletion, so the same button rescues a lapsed instance.

Renewal is manual because it is the entire reclaim signal: an instance
nobody renews is one nobody is using, which is what makes reaping safe.

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

admin/internal/mail/mail.go

@@ -73,3 +73,10 @@ func SendInstanceReady(to, instanceName, loginURL string, expires time.Time) err
 		expires.Format("2 January 2006"))
 	return send(to, instanceName+" is ready", body)
 }
+
+// SendRenewed confirms a renewal and states the new date.
+func SendRenewed(to, instanceName string, expires time.Time) error {
+	return send(to, instanceName+" renewed",
+		fmt.Sprintf("%s is renewed.\n\nYour Free licence now runs until %s.\n",
+			instanceName, expires.Format("2 January 2006")))
+}