(Updates with black cat and quote from Russian fan) By Antonio Denti and Cristia** Corvi** VENICE, Sept 29 (Reuters) - American screen idol George Clooney and Lebanese-born human rights lawyer Amal Alamuddin formalised their marriage in a civil ceremony in Venice on Monday, after a weekend of celebrations attended by Hollywood stars. The couple, who married at a lavish party on Saturday night, spent about 10 minutes inside the lagoon city's 14th century city hall for a civil service registering their marriage. It was presided over by former Rome mayor and close friend, Walter Veltroni. ...