“Casablanca,’’ which celebrates its 70th birthday today with a screening at the Fenway 13 and theaters across the country, needs no introduction. Movie fans already know how it set the standard for big-star acting (by Humphrey Bogart and Ingrid Bergman) and character acting (by Claude Rains, Sydney Greenstreet, and many others). How its iconic scenes, including its airport finale, have become embedded in popular culture, featured on greeting cards and film montages. How its screenplay spawned numerous catchphrases.