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.sqlite/20240806182921_test_schema.sql

@@ -14,3 +14,19 @@ create table models (
 create unique index uix_models_on_provider_id_name on models (provider_id, name);
 create index ix_models_on_provider_id on models (provider_id);
 create index ix_models_on_name on models (name);
+
+create table if not exists usages (
+    id integer primary key autoincrement,
+    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 integer not null default 0,
+    requests_this_day integer not null default 0,
+    tokens_this_day integer not null default 0,
+    requests_this_month integer not null default 0,
+    tokens_this_month integer 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);