collab: Add support for a custom monthly allowance for LLM usage (#19525)

This PR adds support for setting a monthly LLM usage allowance for
certain users.

Release Notes:

- N/A

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

@@ -21,6 +21,7 @@ pub struct Model {
     pub metrics_id: Uuid,
     pub created_at: NaiveDateTime,
     pub accepted_tos_at: Option<NaiveDateTime>,
+    pub custom_llm_monthly_allowance_in_cents: Option<i32>,
 }
 
 #[derive(Copy, Clone, Debug, EnumIter, DeriveRelation)]