Signup funnel events for a mobile app

Design event collection and a daily funnel report for a fitness app with 2 million installs. The product team wants to know, per day, app version and country, how many people reached each step of signup: app opened, signup started, email entered, account created, first workout started. The app logs these events on the device and sends them in batches; phones go offline, retry uploads, and have wrong clocks. About 1.5 million app sessions a day produce around 20 million events. Describe the event format, the collection endpoint, storage, and how the daily funnel is computed so that duplicates and late events do not distort it.