feat: add monitoring agent (Go) + agent report API + cloud billing UI

- Go agent: CPU/mem/disk/port collectors, HTTP report, static binary
- Cross-compile: linux/amd64 + linux/arm64 (statically linked)
- One-line install script for all Linux distros
- Backend: POST /api/v1/agent/report (no JWT) + GET /agent/latest,history (JWT)
- CloudBilling.tsx: fixed syntax errors, stats/trends/records/alerts
- K8s menu renamed to Kubernetes管理

Signed-off-by: Logan.Li

backend/pkg/database/database.go

@@ -91,5 +91,6 @@ func AutoMigrate(db *gorm.DB) error {
 		&models.ResourceModel{},
 		&models.CloudBilling{},
 		&models.BillingAlertRule{},
+		&models.AgentReport{},
 	)
 }