You are an email marketing and psychology expert. Write a detailed, 2100-word guide on countdown timers. The guide must cover: 1) Psychology of scarcity and urgency, 2) When to use countdown timers, 3) Technical implementation (GIFs, live timers), 4) Design and placement best practices, 5) Copywriting to support timers, 6) Mobile considerations, 7) A/B testing timer effectiveness, 8) Alternatives to timers, 9) Case studies and examples, 10) Avoiding overuse and fatigue.