collab: Add LLM request overage columns to billing_preferences (#29446)

This PR adds two new columns to the `billing_preferences` table to allow
users to opt in to overages on LLM requests.

Release Notes:

- N/A

crates/collab/migrations/20250425201930_add_model_request_overages_to_billing_preferences.sql

@@ -0,0 +1,3 @@
+alter table billing_preferences
+    add column model_request_overages_enabled bool not null default false,
+    add column model_request_overages_spend_limit_in_cents integer not null default 0;

crates/collab/src/db/tables/billing_preference.rs

@@ -9,6 +9,8 @@ pub struct Model {
     pub created_at: DateTime,
     pub user_id: UserId,
     pub max_monthly_llm_usage_spending_in_cents: i32,
+    pub model_request_overages_enabled: bool,
+    pub model_request_overages_spend_limit_in_cents: i32,
 }
 
 #[derive(Copy, Clone, Debug, EnumIter, DeriveRelation)]