Add actionBy to merge actions

[epipav]

Changes proposed ✍️

Adds actionBy field to mergeActions table, for referencing which user made the merge action

What

copilot:summary ​ copilot:poem

Why

How

copilot:walkthrough

Checklist ✅

• [x] Label appropriately with Feature, Improvement, or Bug.
• [ ] Add screenshots to the PR description for relevant FE changes
• [ ] New backend functionality has been unit-tested.
• [ ] API documentation has been updated (if necessary) (see docs on API documentation).
• [x] Quality standards are met.