fix: stabilize inbox confirmation retries

backend/internal/models/inbox_item.go

@@ -13,6 +13,7 @@ type SenlinAgentInboxItem struct {
 	Title             string
 	Body              string
 	Status            string `gorm:"not null;default:open"`
+	ConfirmedCount    int    `gorm:"not null;default:0"`
 	CreatedAt         time.Time
 	UpdatedAt         time.Time
 }