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/global_traffic_postgres_test.go

@@ -0,0 +1,39 @@
+package service
+
+import (
+	"os"
+	"testing"
+
+	"gorm.io/gorm"
+
+	"github.com/mhsanaei/3x-ui/v3/internal/testpg"
+)
+
+func initGlobalHeartbeatDB(t *testing.T) *gorm.DB {
+	t.Helper()
+	cleanup, err := testpg.IsolatePackage("global_heartbeat")
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(cleanup)
+	return initTrafficTestDB(t)
+}
+
+func TestGlobalTrafficHeartbeatPostgres(t *testing.T) {
+	if os.Getenv("XUI_DB_TYPE") != "postgres" || os.Getenv("XUI_DB_DSN") == "" {
+		t.Skip("set XUI_DB_TYPE=postgres and XUI_DB_DSN")
+	}
+	for _, tc := range []struct {
+		name string
+		run  func(*testing.T)
+	}{
+		{"bounds", TestGlobalTrafficHeartbeatBounds},
+		{"quota and lifecycle", TestGlobalTrafficHeartbeatQuotaAndLifecycle},
+		{"forwarding", TestGlobalTrafficHeartbeatForwarding},
+		{"nullable and batches", TestGlobalTrafficHeartbeatNullableAndBatch},
+		{"long idle and recovery", TestGlobalTrafficHeartbeatLongIdleAndRecovery},
+		{"concurrent pairs", TestGlobalTrafficHeartbeatConcurrentPairs},
+	} {
+		t.Run(tc.name, tc.run)
+	}
+}