Create a Discord scheduled event

Create a guild scheduled event. Three event types, selected via the discriminator on entity.type:

• external — off-platform (Zoom, in-person, livestream). Requires both location and endsAt. Most common type for scheduler integrations.
• voice — hosted in a Discord voice channel. Requires channelId.
• stage — hosted in a Discord stage channel. Requires channelId.

Bot needs MANAGE_EVENTS in the guild. Existing installs (pre-events PR) need a re-invite OR a server admin manually granting the permission — see route header for details.