A 68-year-old man with known stable angina presents with an episode of central chest pain lasting 30 minutes, occurring at rest, and relieved by glyceryl trinitrate (GTN) spray. His ECG taken during pain shows ST depression in leads V4-V6, which resolves after the pain subsides. Troponin T level is elevated at 90 ng/L (upper limit of normal <14 ng/L). What is the most likely diagnosis?