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

crates/collab/src/llm/db/queries.rs

@@ -2,4 +2,5 @@ use super::*;
 
 pub mod billing_events;
 pub mod providers;
+pub mod subscription_usages;
 pub mod usages;