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/lib.rs

@@ -174,7 +174,7 @@ pub struct Config {
     pub slack_panics_webhook: Option<String>,
     pub auto_join_channel_id: Option<ChannelId>,
     pub stripe_api_key: Option<String>,
-    pub stripe_price_id: Option<Arc<str>>,
+    pub stripe_llm_usage_price_id: Option<Arc<str>>,
     pub supermaven_admin_api_key: Option<Arc<str>>,
     pub user_backfiller_github_access_token: Option<Arc<str>>,
 }
@@ -193,6 +193,10 @@ impl Config {
         }
     }
 
+    pub fn is_llm_billing_enabled(&self) -> bool {
+        self.stripe_llm_usage_price_id.is_some()
+    }
+
     #[cfg(test)]
     pub fn test() -> Self {
         Self {
@@ -231,7 +235,7 @@ impl Config {
             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,
         }