Gitea Actions task token private repository enumeration

Gitea Actions task tokens authenticate to the REST API as a shared system user with a negative ID, which repository, organization, project and key filters treated as having no owner restriction, and the token carried no API scope to restrict user-scoped routes. A user who can run a workflow could use the job's token with `GET /api/v1/user/repos` to list every repository on the instance, including private repositories of other users and organizations, with their metadata. Repository contents remained protected by per-repository permission checks. System users now own nothing, and `/api/v1/user` routes other than `GET /api/v1/user` accept only individual and bot accounts.