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/queries/billing_customers.rs

@@ -11,6 +11,7 @@ pub struct UpdateBillingCustomerParams {
     pub user_id: ActiveValue<UserId>,
     pub stripe_customer_id: ActiveValue<String>,
     pub has_overdue_invoices: ActiveValue<bool>,
+    pub trial_started_at: ActiveValue<Option<DateTime>>,
 }
 
 impl Database {
@@ -45,7 +46,8 @@ impl Database {
                 user_id: params.user_id.clone(),
                 stripe_customer_id: params.stripe_customer_id.clone(),
                 has_overdue_invoices: params.has_overdue_invoices.clone(),
-                ..Default::default()
+                trial_started_at: params.trial_started_at.clone(),
+                created_at: ActiveValue::not_set(),
             })
             .exec(&*tx)
             .await?;

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,
 }