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.

database/model/node_client_traffic.go

@@ -0,0 +1,9 @@
+package model
+
+type NodeClientTraffic struct {
+	Id     int    `json:"id" gorm:"primaryKey;autoIncrement"`
+	NodeId int    `json:"nodeId" gorm:"uniqueIndex:idx_node_email,priority:1;not null"`
+	Email  string `json:"email" gorm:"uniqueIndex:idx_node_email,priority:2;not null"`
+	Up     int64  `json:"up"`
+	Down   int64  `json:"down"`
+}