Is it possible to add a lock feature to prevent task edits after assignment, as well as an audit trail to log all changes made to tasks?

Is it possible to add a lock feature to prevent task edits after assignment, as well as an audit trail to log all changes made to tasks?

Also I am unable to add pdf files as an attachment whereas jpg files and docx files can be uploaded.

_Originally posted by @Nero-devbot in https://github.com/basecamp/fizzy/discussions/2931_

The issue combines task locking, change auditing, and a separate PDF attachment problem. Start by reproducing the PDF upload failure and reviewing the existing task assignment, task editing, and attachment flows. Done would require the scope and expected behavior for all three requests to be agreed and covered by tests.

Written by the indexing model from the issue text.