fix(node): drop online clients and sub-nodes of nodes no longer synced

What the master derives from a node's reports (online clients, active
inbounds, learned sub-nodes) must live only while that node is still
synced; ClearNodeOnlineClients states it: a downed node must not keep its
clients listed as online.

Only a failed snapshot fetch cleared the online set, and only a failed
probe cleared sub-nodes. A disabled node (both jobs skip it), a node marked
offline before the sync tick reached it, a deleted node, and a node whose
snapshot fetched but failed to merge all kept their clients online in
onlineClients, onlineByGuid and activeInbounds, which the dashboard and a
parent master's /clients/onlines read. Disabled and deleted nodes also kept
their sub-nodes on the Nodes page until the panel restarted.

The traffic sync now keeps online sets only for enabled, online nodes in
its list, the heartbeat keeps sub-nodes only for enabled listed nodes, both
before the empty-list return, and a failed merge clears like a failed
fetch. The sync job's one-line call has no job-level test: that package
cannot install the xray process, so RetainSyncedNodeOnlineClients carries
the tested rule.

internal/web/service/inbound_node.go

@@ -355,6 +355,10 @@ func (s *InboundService) SetRemoteTraffic(nodeID int, snap *runtime.TrafficSnaps
 		structuralChange, inner = s.setRemoteTrafficLocked(nodeID, snap, dirty, justPushed)
 		return inner
 	})
+	if err != nil {
+		// As on a failed fetch: a node whose snapshot did not merge keeps no online set.
+		s.ClearNodeOnlineClients(nodeID)
+	}
 	return structuralChange, err
 }
 
@@ -1454,6 +1458,20 @@ func (s *InboundService) ClearNodeOnlineClients(nodeID int) {
 	}
 }
 
+// RetainSyncedNodeOnlineClients keeps online clients only for nodes the traffic
+// sync still fetches; a node missing from nodes was deleted.
+func (s *InboundService) RetainSyncedNodeOnlineClients(nodes []*model.Node) {
+	process := currentXrayProcess()
+	if process == nil {
+		return
+	}
+	synced := make(map[int]bool, len(nodes))
+	for _, n := range nodes {
+		synced[n.Id] = n.Enable && n.Status == "online"
+	}
+	process.RetainNodeOnlineClients(func(nodeID int) bool { return synced[nodeID] })
+}
+
 // panelGuid returns this panel's stable self-identifier, used to key the local
 // panel's own clients in the per-node online maps (#4983).
 func (s *InboundService) panelGuid() string {