Add challenge voting endpoint

[MaryammAli]

Description: Add API for community voting on weekly challenges.
- Path: BackendAcademy/src/challenges/

[DIFoundation]

I'd like to work on this issue. I have experience building REST APIs with NestJS and implementing backend features with validation, clean architecture, and testing. I'll add the challenge voting endpoint under BackendAcademy/src/challenges, integrate it with the existing module structure, handle validation and edge cases, and ensure it aligns with the project's conventions.