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/llm/db.rs

@@ -97,6 +97,14 @@ impl LlmDatabase {
             .ok_or_else(|| anyhow!("unknown model {provider:?}:{name}"))?)
     }
 
+    pub fn model_by_id(&self, id: ModelId) -> Result<&model::Model> {
+        Ok(self
+            .models
+            .values()
+            .find(|model| model.id == id)
+            .ok_or_else(|| anyhow!("no model for ID {id:?}"))?)
+    }
+
     pub fn options(&self) -> &ConnectOptions {
         &self.options
     }