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/tables.rs

@@ -2,5 +2,6 @@ pub mod billing_event;
 pub mod model;
 pub mod monthly_usage;
 pub mod provider;
+pub mod subscription_usage;
 pub mod usage;
 pub mod usage_measure;