Six years after dire financial straits had Cape Cod Hospital looking for a buyer, the community hospital is back on its feet – treating a huge influx of patients in the emergency room during the summer and expanding thanks to well-heeled donors who want reliable health care on their side of the Sagamore Bridge. Some 330 people visit the Hyannis emergency department daily between Memorial Day and Labor Day when the Cape's population nearly triples and the hospital becomes the default destination for anyone who gets sick. The volume the emergency room sees during the summer represents a 40 percent increase over the off-season.