feat(admin): subscription webhooks promote the entitlement and reissue; cancel and past-due take no licence action

created/updated/activated fold into 'make the world match current state', so
out-of-order delivery is correct by construction. A confirmed subscription
promotes desired->granted and signs from granted only. Cancel and past-due touch
only status; the licence runs to expiry. IssueForInstance backstops a
self-hosted placeholder that is linked after payment.

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

admin/internal/mail/mail.go

@@ -164,6 +164,32 @@ func SendLicense(to, instanceName, blob string) error {
 			instanceName, blob))
 }
 
+// SendCancelled confirms a cancellation and states what stays true: the licence
+// keeps working until it expires, then the instance degrades to read-only.
+func SendCancelled(to, instanceName string) error {
+	return send(to, "Your Vantage subscription is cancelled",
+		fmt.Sprintf("Your subscription for %s is cancelled.\n\n"+
+			"Your instance keeps working until the current licence expires. After "+
+			"that, monitors keep running but changes are disabled.\n", instanceName))
+}
+
+// SendPastDue notifies of a failed charge without alarming: the licence is
+// untouched while Paddle retries the card.
+func SendPastDue(to, instanceName string) error {
+	return send(to, "Payment failed for your Vantage subscription",
+		fmt.Sprintf("A payment for %s failed.\n\n"+
+			"Your instance is unaffected while the card is retried. Update your "+
+			"payment method from the billing portal.\n", instanceName))
+}
+
+// SendLinkReminder chases a self-hosted customer who paid but never linked.
+func SendLinkReminder(to, instanceName string) error {
+	return send(to, "Finish setting up "+instanceName,
+		fmt.Sprintf("Your subscription for %s is active, but the instance is not "+
+			"linked yet.\n\nPaste your install's ID in the portal to receive your "+
+			"licence.\n", instanceName))
+}
+
 // SendInstanceReady tells a customer their cloud instance exists, where it is,
 // and when its licence runs out.
 //