collab: Update billing code for LLM usage billing (#18879)

This PR reworks our existing billing code in preparation for charging
based on LLM usage.

We aren't yet exercising the new billing-related code outside of
development.

There are some noteworthy changes for our existing LLM usage tracking:

- A new `monthly_usages` table has been added for tracking usage
per-user, per-model, per-month
- The per-month usage measures have been removed, in favor of the
`monthly_usages` table
- All of the per-month metrics in the Clickhouse rows have been changed
from a rolling 30-day window to a calendar month

Release Notes:

- N/A

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Richard <richard@zed.dev>
Co-authored-by: Max <max@zed.dev>

crates/collab/migrations_llm/20241008155620_create_monthly_usages.sql

@@ -0,0 +1,13 @@
+create table monthly_usages (
+    id serial primary key,
+    user_id integer not null,
+    model_id integer not null references models (id) on delete cascade,
+    month integer not null,
+    year integer not null,
+    input_tokens bigint not null default 0,
+    cache_creation_input_tokens bigint not null default 0,
+    cache_read_input_tokens bigint not null default 0,
+    output_tokens bigint not null default 0
+);
+
+create unique index uix_monthly_usages_on_user_id_model_id_month_year on monthly_usages (user_id, model_id, month, year);