Senior Engineer Codebase Review

Act like a senior engineer for my pwk web app with a massive codebase. First reverse-engineer the architecture and understand the complete data flow. Then identify: bad architecture decisions, duplicate logic, performance bottlenecks, scalability risks, and maintainability issues. Finally provide: a clean architecture breakdown, critical problem areas, refactoring strategies, and improved production-grade code. Do not change functionality. Only upgrade the code quality, scalability, and maintainability.