Folder renames in the database are not reflected in psql

When a folder gets renamed in the source database folder, a new entry is created in psql, with the old one breaking (because pointing to the old slug folder name, not existing anymore). `sync_db` should be aware of renames.

Start by locating the sync_db entry point and tracing how source database folders become psql entries. Reproduce a folder rename and inspect how the old slug and new entry are handled. Done means a rename updates or reconciles the existing psql entry without leaving the old slug broken.

Written by the indexing model from the issue text.