Add edit and delete to calendar events

## Problem

Both the API and the UI support creating a scheduled calendar event, but neither has an edit or delete path once one exists. If a meeting time changes or a session gets cancelled, there's currently no way to update or remove the existing `CalendarEvent` — only create a new one, leaving the stale entry behind.

## Proposed solution

- Add `PATCH /v1/orgs/:orgId/calendar/:eventId` and `DELETE /v1/orgs/:orgId/calendar/:eventId` to `apps/api`, gated through the same ABAC checks the existing `POST` route uses.
- Add edit and delete actions to `calendar-view.tsx`, following the same modal pattern already used for creating an event.
- Add regression tests covering both the happy path and the ABAC boundary (a caller without schedule permission shouldn't be able to edit or delete someone else's event).

## References

- [`docs/roadmap.md`](https://github.com/hoangsonww/Threadline-RealTime-Collab/blob/main/docs/roadmap.md) — "Calendar events are create-only"
- [`docs/api.md#organizations--rooms`](https://github.com/hoangsonww/Threadline-RealTime-Collab/blob/main/docs/api.md)