Add unit tests for endpoints (handlers)

[0xtrr]

We could consider to write unit tests for our REST API. I'm not sure if it makes any sense or if it just increases the codebase or maintenance work but I'm adding it nonetheless. I don't know how these types of tests are structured in Rust or with Axum (API dependency) so I'll just leave this task here for future me or if anyone else wants to take a stab at it.