fix: 恢复腾讯云账单同步并完善成本预测

- 接入腾讯云单日账单、全局限频和指数退避

- 修正月末费用预测并增加账期提示和费用归因

- 补充腾讯云限频及账单归因回归测试

backend/controller/billing_analysis_test.go

@@ -0,0 +1,45 @@
+package controller
+
+import (
+	"testing"
+
+	"ezops/model"
+
+	"github.com/glebarez/sqlite"
+	"gorm.io/gorm"
+)
+
+// TestBillingDimensionAttribution 验证费用变化额和贡献度使用数据库完整数据计算。 // Logan.Li
+func TestBillingDimensionAttribution(t *testing.T) {
+	db, err := gorm.Open(sqlite.Open(":memory:"), &gorm.Config{})
+	if err != nil {
+		t.Fatalf("打开测试数据库失败: %v", err)
+	}
+	if err = db.AutoMigrate(&model.CloudBilling{}); err != nil {
+		t.Fatalf("迁移账单表失败: %v", err)
+	}
+	records := []model.CloudBilling{
+		{BillingDate: "2026-06-01", Provider: "tencent", Service: "CVM", Region: "广州", AccountName: "预发", Currency: "CNY", Cost: 100},
+		{BillingDate: "2026-07-01", Provider: "tencent", Service: "CVM", Region: "广州", AccountName: "预发", Currency: "CNY", Cost: 180},
+		{BillingDate: "2026-07-01", Provider: "tencent", Service: "COS", Region: "广州", AccountName: "预发", Currency: "CNY", Cost: 20},
+	}
+	if err = db.Create(&records).Error; err != nil {
+		t.Fatalf("写入测试账单失败: %v", err)
+	}
+
+	handler := NewBillingHandler(db)
+	items := handler.billingDimensionAttribution("service", "2026-06-01", "2026-06-30", "2026-07-01", "2026-07-31", "CNY", 100)
+	if len(items) != 2 {
+		t.Fatalf("归因项数量错误: got=%d want=2", len(items))
+	}
+	result := map[string]billingAttribution{}
+	for _, item := range items {
+		result[item.Name] = item
+	}
+	if result["CVM"].Delta != 80 || result["CVM"].Contribution != 80 {
+		t.Fatalf("CVM 归因错误: %+v", result["CVM"])
+	}
+	if result["COS"].Delta != 20 || result["COS"].Contribution != 20 {
+		t.Fatalf("COS 归因错误: %+v", result["COS"])
+	}
+}