perf(traffic): skip unchanged global writes between heartbeats (#6931)

* perf(traffic): skip unchanged global writes between heartbeats

Persist directional usage changes immediately, including lower resets, while
refreshing identical global totals only at an inclusive one-hour heartbeat.
Use the GORM clock consistently for the existing 24-hour freshness window and
preserve local-only snapshots, quota enforcement, and downstream forwarding.

Fixes #6928

* docs(traffic): describe persisted global heartbeat timestamps

Replace the stale every-push model comment with the change-or-hourly-heartbeat
rule. Keep the local-counter separation, delta accounting, MAX fold and reset
semantics explicit without treating UpdatedAt as a last-push timestamp.

Address the nonblocking review feedback on PR #6931 without rewriting
published history.

Refs #6928

internal/web/service/client_paging.go

@@ -185,7 +185,7 @@ func newClientQuery(db *gorm.DB, nowMs, expireDiffMs, trafficDiffBytes int64) cl
 		joins:            []clientQueryJoin{{sql: "LEFT JOIN client_traffics ct ON ct.email = c.email"}},
 		usedExpr:         "(COALESCE(ct.up, 0) + COALESCE(ct.down, 0))",
 	}
-	freshSince := globalTrafficFreshSince()
+	freshSince := globalTrafficFreshSince(db.NowFunc())
 	var probe int64
 	err := db.Model(&model.ClientGlobalTraffic{}).
 		Where("updated_at >= ?", freshSince).