[api] support event onboarding, participant management, and adding staff

requested for haven: enable organizers to set up and manage events programmatically from airtable.

creating an event in a series is already available, as noted in the email discussion. extend the api to cover the remaining event onboarding/setup steps, updating and deleting participants, and adding staff. check existing endpoint coverage and implement the missing operations.

acceptance criteria:

- the event onboarding/setup steps can be completed through the api.
- participants can be updated and deleted through the api.
- staff can be added to an event through the api.
- operations enforce the appropriate token permissions and event/series scope.
- update the api reference with supported fields, authentication requirements, and request/response examples for the new operations.

existing event creation reference: https://attend.hackclub.com/docs/api-reference/series/create-an-event-in-the-series

context: haven / attend email discussion, 10 september 2026. the intended use is automated event setup from airtable.

Start by reviewing the existing event-creation endpoint coverage and the linked API reference, then identify the missing onboarding, participant, and staff operations. Verify the new operations against the stated token-permission and event/series-scope requirements. Done means the API supports all listed actions and the API reference documents fields, authentication, and request/response examples.

Written by the indexing model from the issue text.