Fix assignRecruitersToApplication bug that overwrites existing recruiter assignments

[madisonweiss]

Description: In applications.service.ts, the assignRecruitersToApplication() method has a critical bug where adding new recruiters to an application completely wipes out the existing assigned recruiters instead of appending to them. This causes data loss (eek not good)!! The method needs to be refactored to fetch the existing assigned recruiters and combine them with the new ones (with proper duplicate checking → we shouldn’t be able to assign the same recruiter to an application twice).

Acceptance Criteria:

• New recruiters are combined with existing recruiters without data loss
• Duplicate recruiter assignments are detected and prevented
• Sufficient test coverage of assignRecruitersToApplication (+ duplicate handling tested!)

Effort Estimate:

• T-shirt size: S