A 48-year-old man came to a clinic for oral cavity sanation. He has a history of mild diabetes mellitus. Objective examination reveals whitish papulae against the background of normal buccal mucosa. The papulae are slightly raised above the mucosa. They are located symmetrically, predominantly in the olar space, and form a lace-like pattern. On the upper jaw there are two soldered dental bridges, tooth 37 has an amalgam filling, tooth 36 is covered with a cast metal crown. What is the most likely diagnosis?