collab: Rework Stripe event processing (#15510)

This PR reworks how we process Stripe events for reconciliation
purposes.

The previous approach in #15480 turns out to not be workable, on account
of the Stripe event IDs not being strictly in order. This meant that we
couldn't reliably compare two arbitrary event IDs and determine which
one was more recent.

This new approach leans on the guidance that Stripe provides for
webhooks events:

> Webhook endpoints might occasionally receive the same event more than
once. You can guard against duplicated event receipts by logging the
[event IDs](https://docs.stripe.com/api/events/object#event_object-id)
you’ve processed, and then not processing already-logged events.
>
> https://docs.stripe.com/webhooks#handle-duplicate-events

We now record processed Stripe events in the `processed_stripe_events`
table and use this to filter out events that have already been
processed, so we do not process them again.

When retrieving events from the Stripe events API we now buffer the
unprocessed events so that we can sort them by their `created` timestamp
and process them in (roughly) the order they occurred.

Release Notes:

- N/A

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

@@ -9,7 +9,6 @@ pub struct Model {
     pub id: BillingCustomerId,
     pub user_id: UserId,
     pub stripe_customer_id: String,
-    pub last_stripe_event_id: Option<String>,
     pub created_at: DateTime,
 }
 

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

@@ -10,7 +10,6 @@ pub struct Model {
     pub billing_customer_id: BillingCustomerId,
     pub stripe_subscription_id: String,
     pub stripe_subscription_status: StripeSubscriptionStatus,
-    pub last_stripe_event_id: Option<String>,
     pub created_at: DateTime,
 }
 

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

@@ -0,0 +1,16 @@
+use sea_orm::entity::prelude::*;
+
+#[derive(Clone, Debug, PartialEq, DeriveEntityModel)]
+#[sea_orm(table_name = "processed_stripe_events")]
+pub struct Model {
+    #[sea_orm(primary_key)]
+    pub stripe_event_id: String,
+    pub stripe_event_type: String,
+    pub stripe_event_created_timestamp: i64,
+    pub processed_at: DateTime,
+}
+
+#[derive(Copy, Clone, Debug, EnumIter, DeriveRelation)]
+pub enum Relation {}
+
+impl ActiveModelBehavior for ActiveModel {}