collab: Add `GET /billing/usage` endpoint (#28832)

This PR adds a `GET /billing/usage` endpoint for retrieving billing
usage to show on the `zed.dev/account` page.

Release Notes:

- N/A
b486e32f05Marshall Bowers committed on 4/16/2025, 3:28:09 AM· committed by GitHubparent222d4a2
7 files changedLine totals unavailable