Discuss db schema changes

We have some changes planned for the db schema that were already documented in [Proposed DB Changes](https://docs.google.com/document/d/1_YkvFsu7YnmA_2ABPXB4fG5vRRklT3EY3GvhbUEkfBI/edit?usp=sharing) and [KCIDB Roadmap](https://www.google.com/url?q=https://docs.google.com/document/d/16ZNepP_YfVsBDw37Tw-2hneS0y8NGFV_JywpWsHyiS8/edit?tab%3Dt.0&sa=D&source=docs&ust=1753387831038045&usg=AOvVaw3yiJaD44Le6b_NXR-Xvaoq).

This issue includes reviewing those documents and checking which changes makes sense; we will then create an issue for each one of them separately.

We should discuss each bullet point whenever we are ready for those changes.

Start by reviewing the linked Proposed DB Changes and KCIDB Roadmap documents. Discuss whether each listed schema change makes sense, record the decisions, and create a separate issue for every accepted change; no source file or test entry point is named in this issue.

Written by the indexing model from the issue text.