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/node_tree.go

@@ -86,6 +86,22 @@ func (s *NodeService) ClearDescendants(nodeID int) {
 	nodeDescendantsMu.Unlock()
 }
 
+// RetainEnabledNodeDescendants drops sub-nodes learned from nodes the heartbeat no
+// longer probes: disabled ones it skips, deleted ones missing from nodes.
+func (s *NodeService) RetainEnabledNodeDescendants(nodes []*model.Node) {
+	enabled := make(map[int]bool, len(nodes))
+	for _, n := range nodes {
+		enabled[n.Id] = n.Enable
+	}
+	nodeDescendantsMu.Lock()
+	for nodeID := range nodeDescendantsCache {
+		if !enabled[nodeID] {
+			delete(nodeDescendantsCache, nodeID)
+		}
+	}
+	nodeDescendantsMu.Unlock()
+}
+
 func cachedDescendants() []model.NodeSummary {
 	nodeDescendantsMu.RLock()
 	defer nodeDescendantsMu.RUnlock()