collab: Prevent users from ending up with multiple active billing subscriptions (#19574)

This PR adds some safeguards to ensure that users do not end up with
multiple active billing subscriptions.

We now do the following:

1. When initiating a checkout, we first make sure the user does not
already have an active subscription.
2. When creating subscriptions in response to Stripe events, we ensure
that we don't already have an active subscription.

Release Notes:

- N/A

crates/collab/src/api/billing.rs

@@ -226,6 +226,13 @@ async fn create_billing_subscription(
         ))?
     };
 
+    if app.db.has_active_billing_subscription(user.id).await? {
+        return Err(Error::http(
+            StatusCode::CONFLICT,
+            "user already has an active subscription".into(),
+        ));
+    }
+
     let customer_id =
         if let Some(existing_customer) = app.db.get_billing_customer_by_user_id(user.id).await? {
             CustomerId::from_str(&existing_customer.stripe_customer_id)
@@ -655,6 +662,33 @@ async fn handle_customer_subscription_event(
             )
             .await?;
     } else {
+        // If the user already has an active billing subscription, ignore the
+        // event and return an `Ok` to signal that it was processed
+        // successfully.
+        //
+        // There is the possibility that this could cause us to not create a
+        // subscription in the following scenario:
+        //
+        //   1. User has an active subscription A
+        //   2. User cancels subscription A
+        //   3. User creates a new subscription B
+        //   4. We process the new subscription B before the cancellation of subscription A
+        //   5. User ends up with no subscriptions
+        //
+        // In theory this situation shouldn't arise as we try to process the events in the order they occur.
+        if app
+            .db
+            .has_active_billing_subscription(billing_customer.user_id)
+            .await?
+        {
+            log::info!(
+                "user {user_id} already has an active subscription, skipping creation of subscription {subscription_id}",
+                user_id = billing_customer.user_id,
+                subscription_id = subscription.id
+            );
+            return Ok(());
+        }
+
         app.db
             .create_billing_subscription(&CreateBillingSubscriptionParams {
                 billing_customer_id: billing_customer.id,