[Feature] option to merge annotation labels

[rasa-pa]

📄 Description

In typical workflows, we begin with an initial list of defect types provided as part of the project requirements. However, during model development, it often becomes clear that the model struggles to reliably distinguish between certain defects that belong to the same broader category, for example, scratch_bright and scratch_dark.
When this happens, the practical solution is to combine these defects under a single label. Today, we do it manually, update annotations image by image.

🎯 Objective

Option to merge two or more labels into one so that defect schemas can be adapted quickly based on model results, and iteration cycles become faster and more efficient.