Christopher Vourlias “When business in brothels is slow, that means the economy is on the ropes.” Those words, uttered by the indelible madame at the heart of director Eva Stefani’s “Days and Nights of Demetra K.,” offers a fitting introduction to the way in which the ups and downs in the life of a single sex worker mirror the larger political and economic uncertainties rattling post-crisis Greece.Stefani’s feature-length debut, which has its premiere in the main competition at the Thessaloniki