feat: Move mail system to shared

mail/monitor.go

@@ -0,0 +1,26 @@
+package mail
+
+import "time"
+
+// MonitorEvent is a monitor state transition, as the control plane's
+// notification dispatcher sees it. It lives here rather than in server/ so that
+// the email templates and the caller agree on the fields without server's model
+// package leaking into shared.
+type MonitorEvent struct {
+	MonitorName string
+	Type        string
+	OldStatus   string
+	NewStatus   string
+	Message     string
+	Time        time.Time
+
+	// Down drives the pill and the subject verb. The caller decides it, since
+	// only server/ knows which status strings mean down.
+	Down bool
+}
+
+// SendMonitorAlert delivers one state-change notification to an SMTP
+// notification channel's recipients, which may be a comma-separated list.
+func (s Sender) SendMonitorAlert(to string, ev MonitorEvent) error {
+	return s.sendTemplate(to, "", "monitoralert", ev)
+}