collab: Transfer existing usage from trial to Pro (#28884)

This PR adds support for transferring any existing usage from a trial
subscription to a Zed Pro subscription when the user upgrades.

Release Notes:

- N/A

---------

Co-authored-by: Mikayla <mikayla@zed.dev>

crates/collab/src/api/billing.rs

@@ -843,6 +843,28 @@ async fn handle_customer_subscription_event(
         .get_billing_subscription_by_stripe_subscription_id(&subscription.id)
         .await?
     {
+        let llm_db = app
+            .llm_db
+            .clone()
+            .ok_or_else(|| anyhow!("LLM DB not initialized"))?;
+
+        let new_period_start_at =
+            chrono::DateTime::from_timestamp(subscription.current_period_start, 0)
+                .ok_or_else(|| anyhow!("No subscription period start"))?;
+        let new_period_end_at =
+            chrono::DateTime::from_timestamp(subscription.current_period_end, 0)
+                .ok_or_else(|| anyhow!("No subscription period end"))?;
+
+        llm_db
+            .transfer_existing_subscription_usage(
+                billing_customer.user_id,
+                &existing_subscription,
+                subscription_kind,
+                new_period_start_at,
+                new_period_end_at,
+            )
+            .await?;
+
         app.db
             .update_billing_subscription(
                 existing_subscription.id,