WIP: Clsuter support (cross node delivery, leader election)

cluster/cluster_nop.go

@@ -0,0 +1,24 @@
+package cluster
+
+import (
+	"net/http"
+
+	"heckel.io/ntfy/v2/model"
+)
+
+// nopCluster is the single-node default: it drops all relayed messages, rejects peer API requests, and
+// reports this node as leader (a single node is trivially the leader, so leader-gated jobs need
+// no special-casing in single-node mode).
+type nopCluster struct{}
+
+func (c *nopCluster) Relay(_ *model.Message) error { return nil }
+
+func (c *nopCluster) ServeHTTP(w http.ResponseWriter, _ *http.Request) {
+	w.WriteHeader(http.StatusNotFound)
+}
+
+func (c *nopCluster) AnnounceTopics(_ []string) {}
+
+func (c *nopCluster) IsLeader() bool { return true }
+
+func (c *nopCluster) Close() error { return nil }