TANGI SALAUN THE teenager identified by prosecutors as Abdoulakh A. lived with his family and had never appeared on the radar of French intelligence agencies. But he propelled himself into their sights in a horrific manner on Friday evening when he beheaded a school teacher in broad daylight. The 18-year-old had a record of juvenile delinquency but was too young for police to have built up a file on him. He lived in a Chechen community that is not well understood by intelligence services. He used a knife to decapitate the middle school history teacher, Samuel Paty, who had shown…