feat: cluster reliability (leader failover, crash rejoin, key exchange) + auth/users + canvas/forwards enhancements + comprehensive README + API docs

- Cluster: forwardToNext offline detection (leader+non-leader), WatchLeader 1s heartbeat fallback, 409 for standalone nodes, Node.NodeKey key exchange via token ring, ClusterPeers persistence + auto-rejoin, Forward delegates to forwardToNext (bugfix)
- Auth: Basic Auth (flag-creds fast path) + bcrypt users (admin/viewer) + Bearer API keys (read/write/admin scope)
- Frontend: UsersView (accounts+API keys), ClusterView (ring/nodeKey/tasks/topology/log), StatusView (group management, per-proxy status), CanvasView (edge toggle/group), PortEdge (disabled/group labels)
- API: handlers split (canvas/forwards/users/logs), canvas export/import, forwards group start/stop/assign/delete, cluster endpoints
- Docs: comprehensive README rewrite (all flags/APIs/auth/cluster), docs/cluster-api.md (cluster management API reference)
- Deploy: run-cluster.sh now 4-node ring + 1 isolated standalone, test-forward.sh updated for 4 nodes
- Removed plan.md (design notes consolidated into README + API docs)

internal/httpapi/server_test.go

@@ -211,7 +211,7 @@ func TestSaveCanvasPublishesRevokeTask(t *testing.T) {
 	})
 	ring := cluster.NewEngine("n1", "n1:7500", "u", "p", "0.1.0", nil,
 		&cluster.AppHandler{}, func(ctx context.Context, next string, tk *cluster.Token) error { return nil },
-		"n1:7500", true)
+		"n1:7500", true, "")
 	h := &Handler{Store: st, Process: pm, WorkDir: dir, User: "admin", Password: "pw", Ring: ring}
 	mux, _ := NewServeMux(h)
 	ts := httptest.NewServer(mux)