fix: 按业务时区同步昨日账单

- 固定按 Asia/Shanghai 计算账单昨日日期

- 补充 UTC 服务器时区回归测试

backend/pkg/billingsync/sync_test.go

@@ -0,0 +1,14 @@
+package billingsync
+
+import (
+	"testing"
+	"time"
+)
+
+// TestBillingYesterdayAt 验证 UTC 服务器按中国业务时区计算昨日。 // Logan.Li
+func TestBillingYesterdayAt(t *testing.T) {
+	now := time.Date(2026, 7, 28, 21, 30, 0, 0, time.UTC)
+	if got := billingYesterdayAt(now); got != "2026-07-28" {
+		t.Fatalf("昨日日期错误: got=%s want=2026-07-28", got)
+	}
+}