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/migrate_data.go

@@ -36,6 +36,7 @@ func migrationModels() []any {
 		&model.ClientRecord{},
 		&model.ClientInbound{},
 		&model.InboundFallback{},
+		&model.NodeClientTraffic{},
 	}
 }