feat: Move mail system to shared

shared/mail/billing.go

@@ -0,0 +1,18 @@
+package mail
+
+// SendCancelled confirms a cancellation and states what stays true: the licence
+// keeps working until it expires, then the instance degrades to read-only.
+func (s Sender) SendCancelled(to, instanceName string) error {
+	return s.sendTemplate(to, "", "cancelled", struct {
+		InstanceName string
+	}{instanceName})
+}
+
+// SendPastDue notifies of a failed charge without alarming: the licence is
+// untouched while Paddle retries the card.
+func (s Sender) SendPastDue(to, instanceName string) error {
+	return s.sendTemplate(to, "", "pastdue", struct {
+		InstanceName string
+		PortalURL    string
+	}{instanceName, s.PublicURL})
+}