Fitness Tracking App

Build a production-grade fitness platform with workout tracking, exercise libraries, plans, progress analytics, social challenges, and user-specific goals. Define clear models for workout events, plans, measurements, achievements, and challenge participation. Treat body metrics and health-adjacent data as sensitive user information with strict access rules. Include observability for analytics pipelines, auditability for plan and challenge changes, resilient processing for delayed event ingestion, and careful handling of derived metrics so charts and progress summaries remain consistent as historical workout data changes.