BEIJING — Having punished more than a million Communist Party members for corruption, Chinese President Xi Jinping will use a key meeting that started Monday to drive home the message that his signature anti-graft campaign is far from over and that his authority remains undiminished. The Central Committee plenary gathering also sets in motion preparations for next year’s 19th national party congress that will kickoff Xi’s second five-year term as head of the ruling party. This week’s meeting comes as Xi is riding high as China’s most powerful leader since Deng Xiaoping led the country in the 1980s and gaining kudos at home for his assertive foreign policy, including the leveraging of China’s political and economic heft to open up a rift between the Philippines and its longstanding treaty ally, the United States. Yet Xi’s domestic challenges are legion, ranging from slowing economic growth to massive layoffs resulting from the closure of steel and coal mines and other heavy industries in an effort to reduce industrial overcapacity.