collab: Add billing preferences for maximum LLM monthly spend (#18948)

This PR adds a new `billing_preferences` table.

Right now there is a single preference: the maximum monthly spend for
LLM usage.

Release Notes:

- N/A

---------

Co-authored-by: Richard <richard@zed.dev>

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

@@ -0,0 +1,30 @@
+use crate::db::{BillingPreferencesId, UserId};
+use sea_orm::entity::prelude::*;
+
+#[derive(Clone, Debug, Default, PartialEq, Eq, DeriveEntityModel)]
+#[sea_orm(table_name = "billing_preferences")]
+pub struct Model {
+    #[sea_orm(primary_key)]
+    pub id: BillingPreferencesId,
+    pub created_at: DateTime,
+    pub user_id: UserId,
+    pub max_monthly_llm_usage_spending_in_cents: i32,
+}
+
+#[derive(Copy, Clone, Debug, EnumIter, DeriveRelation)]
+pub enum Relation {
+    #[sea_orm(
+        belongs_to = "super::user::Entity",
+        from = "Column::UserId",
+        to = "super::user::Column::Id"
+    )]
+    User,
+}
+
+impl Related<super::user::Entity> for Entity {
+    fn to() -> RelationDef {
+        Relation::User.def()
+    }
+}
+
+impl ActiveModelBehavior for ActiveModel {}