fix: validate and prevent duplicate job applications

[DevHola]

Pull Request

Description

This PR fixes an issue where duplicate job applications could be submitted. It introduces a validation check to prevent multiple applications for the same job by the same user.

Related Issue

Fixes #1277

Type of Change

• fix: Bug fix
• test: Test additions/updates
• chore: Build process or tooling changes

How Has This Been Tested?

• Unit tests
• Integration tests
• Manual tests

Test Evidence

Screenshots (if applicable)

Documentation Screenshots (if applicable)

Checklist

• My code follows the project's coding style
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published
• I have included a screenshot showing all tests passing
• I have included documentation screenshots (if applicable)