Publish alerts to a Sandbar topic and keep a Sandbar monitor for every resource with health checks

A Sandbar topic channel publishes alerts as ntfy-style messages (API key
optional). Under Server, Settings, Hammerhead can create, update and remove a
Sandbar monitor per enabled resource with health checks, linked by resource id
so monitors made by hand are left alone. The key is sealed at rest.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

alerts_api.go

@@ -26,6 +26,9 @@ func preview(ch AlertChannel) string {
 	if ch.Kind == "email" {
 		return ch.Target
 	}
+	if u, err := url.Parse(ch.Target); err == nil && ch.Kind == "topic" {
+		return u.Scheme + "://" + u.Host + "/" + ch.Topic
+	}
 	if u, err := url.Parse(ch.Target); err == nil {
 		return u.Scheme + "://" + u.Host + "/…"
 	}
@@ -95,7 +98,7 @@ func (s *Server) saveChannel(w http.ResponseWriter, r *http.Request, sc alertSco
 		if in.Target == "" {
 			in.Target = existing.Target // blank keeps the saved URL/address
 		}
-		if id != "" && in.Token == "" && in.Kind == "sandbar" {
+		if id != "" && in.Token == "" && (in.Kind == "sandbar" || in.Kind == "topic") {
 			in.Token = existing.Token // blank keeps the saved API key
 		}
 	}