HOUSTON — Driven by last summer’s surge of illegal immigration from Central America, the already large backlog in federal immigration courts has reached an all-time high with more than 445,000 pending cases, according to a new report. As of April, the backlog hit 445,706 cases, a nearly 30 percent increase since the start of the last fiscal year, according to the Transactional Records Access Clearinghouse at Syracuse University. Immigration courts have been overwhelmed since last summer’s influx of more than 68,500 unaccompanied children and about as many family units across the southern border, most from Central America.