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/src/tests/test_server.rs

@@ -677,7 +677,7 @@ impl TestServer {
                 migrations_path: None,
                 seed_path: None,
                 stripe_api_key: None,
-                stripe_price_id: None,
+                stripe_llm_usage_price_id: None,
                 supermaven_admin_api_key: None,
                 user_backfiller_github_access_token: None,
             },