collab: Add endpoint for migrating users to new billing (#29769)

This PR adds a new `POST /billing/subscriptions/migrate` endpoint for
migrating users to the new billing system.

When called with a GitHub user ID this endpoint will:

1. Find the active billing subscription for this user (if they have one)
2. Cancel the subscription and send a final invoice
3. Ensure the user is in the `new-billing` and `assistant2` feature
flags

Release Notes:

- N/A

crates/collab/src/api/billing.rs

@@ -54,6 +54,10 @@ pub fn router() -> Router {
             "/billing/subscriptions/manage",
             post(manage_billing_subscription),
         )
+        .route(
+            "/billing/subscriptions/migrate",
+            post(migrate_to_new_billing),
+        )
         .route("/billing/monthly_spend", get(get_monthly_spend))
         .route("/billing/usage", get(get_current_usage))
 }
@@ -610,6 +614,85 @@ async fn manage_billing_subscription(
     }))
 }
 
+#[derive(Debug, Deserialize)]
+struct MigrateToNewBillingBody {
+    github_user_id: i32,
+}
+
+#[derive(Debug, Serialize)]
+struct MigrateToNewBillingResponse {
+    /// The ID of the subscription that was canceled.
+    canceled_subscription_id: String,
+}
+
+async fn migrate_to_new_billing(
+    Extension(app): Extension<Arc<AppState>>,
+    extract::Json(body): extract::Json<MigrateToNewBillingBody>,
+) -> Result<Json<MigrateToNewBillingResponse>> {
+    let Some(stripe_client) = app.stripe_client.clone() else {
+        log::error!("failed to retrieve Stripe client");
+        Err(Error::http(
+            StatusCode::NOT_IMPLEMENTED,
+            "not supported".into(),
+        ))?
+    };
+
+    let user = app
+        .db
+        .get_user_by_github_user_id(body.github_user_id)
+        .await?
+        .ok_or_else(|| anyhow!("user not found"))?;
+
+    let old_billing_subscriptions_by_user = app
+        .db
+        .get_active_billing_subscriptions(HashSet::from_iter([user.id]))
+        .await?;
+
+    let Some((_billing_customer, billing_subscription)) =
+        old_billing_subscriptions_by_user.get(&user.id)
+    else {
+        return Err(Error::http(
+            StatusCode::NOT_FOUND,
+            "No active billing subscriptions to migrate".into(),
+        ));
+    };
+
+    let stripe_subscription_id = billing_subscription
+        .stripe_subscription_id
+        .parse::<stripe::SubscriptionId>()
+        .context("failed to parse Stripe subscription ID from database")?;
+
+    Subscription::cancel(
+        &stripe_client,
+        &stripe_subscription_id,
+        stripe::CancelSubscription {
+            invoice_now: Some(true),
+            ..Default::default()
+        },
+    )
+    .await?;
+
+    let feature_flags = app.db.list_feature_flags().await?;
+
+    for feature_flag in ["new-billing", "assistant2"] {
+        let already_in_feature_flag = feature_flags.iter().any(|flag| flag.flag == feature_flag);
+        if already_in_feature_flag {
+            continue;
+        }
+
+        let feature_flag = feature_flags
+            .iter()
+            .find(|flag| flag.flag == feature_flag)
+            .context("failed to find feature flag: {feature_flag:?}")?;
+
+        app.db.add_user_flag(user.id, feature_flag.id).await?;
+    }
+
+    Ok(Json(MigrateToNewBillingResponse {
+        canceled_subscription_id: stripe_subscription_id.to_string(),
+    }))
+}
+
 /// The amount of time we wait in between each poll of Stripe events.
 ///
 /// This value should strike a balance between: