collab: Add plan column to subscription_usages (#28889)

This PR adds a `plan` column to the `subscription_usages` table.

These tables don't have any records in them yet, so it's fine to make
the column required without a default.

Release Notes:

- N/A

crates/collab/migrations_llm/20250416181354_add_plan_to_subscription_usages.sql

@@ -0,0 +1,4 @@
+alter table subscription_usages
+    add column plan text not null;
+
+create index ix_subscription_usages_on_plan on subscription_usages (plan);