collab: Limit customers to one free trial (#29232)

This PR makes it so customers can only subscribe to the trial once.

Release Notes:

- N/A

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

@@ -10,6 +10,7 @@ pub struct Model {
     pub user_id: UserId,
     pub stripe_customer_id: String,
     pub has_overdue_invoices: bool,
+    pub trial_started_at: Option<DateTime>,
     pub created_at: DateTime,
 }