Rename `id` fields and slugs to more specific names

[asqarslanov]

• Rename REST routes (use #[serde(rename_all = "kebab-case")]):

  • /events/{id}/images to /events/{event-id}/images
  • /events/images/{id} to /events/images/{image-id}
  • so on...

• Update the Protobuf schemas to match the REST schemas.

• (Optionally): rename the domain types.