Context A growing Express application has all business logic mixed directly into route handlers, making it untestable and hard to maintain. The Problem Route files contain database queries, business rules, and HTTP response logic all at the same level, with no separation of concerns. Constraints Separate the codebase into routes, controllers, services, and repositories layers. Each layer communicates only with the one directly below it. Define clear interfaces between layers using dependency injection. Expected Deliverable A refactored Express application following a layered architecture (routes, controllers, services, repositories), demonstrating clear separation of concerns and testable service logic.