R1:3bd0da0ec243

Add a missing table key to improve performance of "Recently Completed Tasks" query

Summary:
Fixes T11490. Currently, this query can not use a key and the table size may be quite large.

Adjust the query so it can use a key for both selection and ordering, and add that key.

Test Plan: Ran `EXPLAIN` on the old query in production, then added the key and ran `EXPLAIN` on the new query. Saw key in use, and "rows" examined drop from 29,273 to 15.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11490…