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.rs

@@ -436,6 +436,9 @@ fn normalize_model_name(known_models: Vec<String>, name: String) -> String {
     }
 }
 
+/// The maximum monthly spending an individual user can reach before they have to pay.
+pub const MONTHLY_SPENDING_LIMIT_IN_CENTS: usize = 5 * 100;
+
 /// The maximum lifetime spending an individual user can reach before being cut off.
 ///
 /// Represented in cents.
@@ -458,6 +461,18 @@ async fn check_usage_limit(
         )
         .await?;
 
+    if state.config.is_llm_billing_enabled() {
+        if usage.spending_this_month >= MONTHLY_SPENDING_LIMIT_IN_CENTS {
+            if !claims.has_llm_subscription.unwrap_or(false) {
+                return Err(Error::http(
+                    StatusCode::PAYMENT_REQUIRED,
+                    "Maximum spending limit reached for this month.".to_string(),
+                ));
+            }
+        }
+    }
+
+    // TODO: Remove this once we've rolled out monthly spending limits.
     if usage.lifetime_spending >= LIFETIME_SPENDING_LIMIT_IN_CENTS {
         return Err(Error::http(
             StatusCode::FORBIDDEN,
@@ -505,7 +520,6 @@ async fn check_usage_limit(
                 UsageMeasure::RequestsPerMinute => "requests_per_minute",
                 UsageMeasure::TokensPerMinute => "tokens_per_minute",
                 UsageMeasure::TokensPerDay => "tokens_per_day",
-                _ => "",
             };
 
             if let Some(client) = state.clickhouse_client.as_ref() {