async processWebmention(webmention) {
        let mention = await this.#repository.getBySourceAndTarget(
            webmention.source,
            webmention.target
        );

        return await this.#updateWebmention(mention, webmention);
    }
@param {object} webmention
@param {URL} webmention.source
@param {URL} webmention.target
@param {Object<string, any>} webmention.payload

@returns {Promise<Mention>}