Gitea: Actions job could fake trusted commit signatures on any repository

The GITEA_TOKEN of a Gitea Actions job authenticates as the gitea-actions system user, and the /api/v1/user email and GPG key endpoints accepted that user, so a job could register an email address and a GPG key on it; without required email confirmation (the default) the address is stored as activated. Commit verification treats a key without a real owner as the instance signing key, so any user who can run a workflow job could make commits signed with their own key show as trusted and signed by the instance in every repository where they appear, including pull requests to repositories the user cannot write to. The impact is limited to the displayed signature status: enforcement such as required signed commits only checks that a signature is verified and is unaffected, and no access is gained. The /api/v1/user routes now reject system users.