Investigate and implement reference-free placeholder user reassignment

Background

Currently, placeholder references are used during the reassignment process for imported contributions. A new approach has been proposed in #521450 that suggests creating references only when using the Import User, not for placeholder users.

Problem

The current system creates references for both Import Users and Placeholder Users. However, with placeholder users, these references are technically unnecessary, as we can already distinguish which contributions require reassignment by matching the placeholder_user_id with the user ID on the contributions.

Goals

In this isssue, we will:

• Validate the technical feasibility of the proposed solution
• Identify potential blockers (e.g., missing database indexes)
• Plan the required implementation work
• Document any technical limitations or concerns discovered

Next Steps

1. Prototype the new reference-free approach
2. Test the reassignment process
3. Document findings and create implementation plan