Add wine and reserved oyster liquor to pot and cook until slightly reduced, about 4 minutes. Add broth and saffron; bring to a boil, reduce heat, and simmer 5 minutes. Add watercress and cook until just wilted, about 2 minutes. Let cool slightly, then, working in batches, purée in a blender until smooth. Pour soup into a large saucepan and bring to simmer over medium heat. Add oysters and red pepper flakes and cook until oysters are just cooked through, about 2 minutes; stir in 1/2 cup cream.