Add ci-base-image with Python 3.14 support
- New Dockerfile.ci-base-image: Ubuntu 24.04 base with Python 3.14 from deadsnakes PPA - Includes full C++ build toolchain, Poetry 2.1.3, Docker CLI, faketime, websocat - Uses hived_admin user (UID 2000) for backwards compatibility with hive/hive ci-base-image - Add build job and cache cleanup entry to .gitlab-ci.yml - Add ci-base-image target to docker-bake.hcl