Cleanup tasks table

There are unnecessary columns in the tasks table.

We should **delete**:
- status
- modified_at

We could **optionally delete**:
- created_at -- provided we insert a received status into task_statuses at the same time.

Locate the tasks table definition and all references to status, modified_at, created_at, and task_statuses. Check the related schema or migration tests before deciding whether created_at can also be removed; done means the required columns are deleted and the optional change is consistently implemented or explicitly left out.

Written by the indexing model from the issue text.