## What changed
Add a migration that renames `thread_artifacts` to `thread_attachments` and
`artifact_type` to `attachment_type`, and recreates the thread lookup index
under the attachment name.
## Testing
Extend the migration test to verify that existing attachment data survives the
rename, section metadata is preserved, and the version 50 migrator tolerates
the updated database with `ignore_missing` enabled.
GitOrigin-RevId: 43955c4f69b91f92217efc1dc8e6a551d0716059
