collab: Keep track of last seen Stripe event for each record (#15480)

This PR improves our Stripe event handling by keeping track of the last
event we've seen for each record.

The `billing_customers` and `billing_subscriptions` tables both have a
new `last_stripe_event_id` column. When we apply an event to one of
these records, we store the event ID that was applied.

Then, when we are going through events we can ignore any event that has
an ID that came before the `last_stripe_event_id` (based on the
lexicographical ordering of the IDs).

Release Notes:

- N/A

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

@@ -9,6 +9,7 @@ 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

@@ -10,6 +10,7 @@ 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,
 }