Unify paid event creation into main event creation

Creating paid events (either as "merch" or "ticketing") should be supported via the events API in the same page that you create an event. The ticketing type and ID should be immutable but other fields should be changeable. This API call should also be able to call the Stripe API to setup prices, etc.

Trace the existing event-creation page and events API entry points first, then identify how paid events are currently handled for merch or ticketing. Confirm the existing Stripe integration and determine the rules for immutable ticketing type and ID versus editable fields. Done means one event-creation flow supports both paid types and can set up Stripe prices.

索引モデルが issue の本文から書いたものです。