Track lifetime spending for each user and model (#16137)

Release Notes:

- N/A

Co-authored-by: Marshall <marshall@zed.dev>

crates/collab/migrations_llm/20240812225346_create_lifetime_usages.sql

@@ -0,0 +1,9 @@
+create table lifetime_usages (
+    id serial primary key,
+    user_id integer not null,
+    model_id integer not null references models (id) on delete cascade,
+    input_tokens bigint not null default 0,
+    output_tokens bigint not null default 0
+);
+
+create unique index uix_lifetime_usages_on_user_id_model_id on lifetime_usages (user_id, model_id);