collab: Add usages table to LLM database (#15884)

This PR adds a `usages` table to the LLM database.

We'll use this to track usage for rate-limiting purposes.

Release Notes:

- N/A

crates/collab/migrations_llm/20240806213401_create_usages.sql

@@ -0,0 +1,15 @@
+create table if not exists usages (
+    id serial primary key,
+    user_id integer not null,
+    model_id integer not null references models (id) on delete cascade,
+    requests_this_minute integer not null default 0,
+    tokens_this_minute bigint not null default 0,
+    requests_this_day integer not null default 0,
+    tokens_this_day bigint not null default 0,
+    requests_this_month integer not null default 0,
+    tokens_this_month bigint not null default 0
+);
+
+create index ix_usages_on_user_id on usages (user_id);
+create index ix_usages_on_model_id on usages (model_id);
+create unique index uix_usages_on_user_id_model_id on usages (user_id, model_id);