Accepts an application/json object with a non-empty `batch` array. Browser requests from allowed origins can omit `api_key`. Backend or no-origin requests require a valid `api_key` from `PUBLIC_API_KEYS`. Unknown top-level and event-level fields are accepted but ignored unless they are inside `properties`. Events missing `event` or a distinct ID are dropped and counted in the response instead of failing the whole request. A distinct ID may be sent as `distinct_id`, `properties.distinct_id`, or `properties.$distinct_id`. Request compression is supported only with `Content-Encoding: gzip`. Other encodings and the `compression` query parameter are rejected.