fix(nodes): sum client traffic across nodes instead of overwriting

A client shared across multiple nodes has a single email-keyed client_traffics row, but each node reports its cumulative up/down. setRemoteTrafficLocked overwrote the row with one node's cumulative, so non-owning nodes hit the create branch and OnConflict-DoNothing, silently dropping their traffic and under-counting the client.

Make the shared row a pure accumulator (like the local path): a new node_client_traffics(node_id, email) baseline table stores each node's last cumulative; the node path converts cumulative to a per-node delta (clamped to the post-reset value on a negative delta) and does up = up + delta. First observation seeds the baseline and adds 0 so upgrades and newly-shared clients are not double-counted. Create-vs-accumulate now keys off global email existence. Baselines are cleaned in DelClientStat, the node sweeps, and NodeService.Delete.

web/service/node.go

@@ -233,6 +233,9 @@ func (s *NodeService) Delete(id int) error {
 	if err := db.Where("id = ?", id).Delete(model.Node{}).Error; err != nil {
 		return err
 	}
+	if err := db.Where("node_id = ?", id).Delete(&model.NodeClientTraffic{}).Error; err != nil {
+		return err
+	}
 	if mgr := runtime.GetManager(); mgr != nil {
 		mgr.InvalidateNode(id)
 	}