[TASK] Persist College and Department IDs from Webtools Events

### Description
Webtools events already include stable organizational identifiers (sponsorUnitId and sponsorDepartmentId), but the backend currently ignores them and only relies on names. This causes College and Department filters to break or require manual updates when units are renamed or reorganized.

Related to: https://github.com/rokwire/illinois-app/issues/5492