Matias Rinne developed and enhanced backend features for the MuViCo/MuViCo repository over a two-month period, focusing on robust access control, validation, and user onboarding. He implemented middleware in Node.js and Express.js to enforce presentation authorization, improved error handling, and broadened validation for presentation data using MongoDB and Mongoose. Matias consolidated validation logic, optimized performance by targeting only modified fields, and expanded security test coverage to ensure proper access and ownership checks. Additionally, he strengthened user signup validation with comprehensive input checks and end-to-end tests, resulting in improved data integrity, security, and reliability across the API and onboarding workflows.