fix(scheduler): report image prune failures (#995)

[014-code]

Summary

• Preserve the exit codes of docker image prune and docker builder prune.
• Mark an ImageCleanupTask worker as failed when either prune command fails.
• Keep docuum startup and existing daemon behavior non-blocking when prune fails.

Issue

Fixes #995

Tests

• Added coverage for Docker prune command failures and failed worker reporting while docuum is already running.
• Passed: .venv\Scripts\python.exe -m pytest tests/unit/admin/scheduler/test_image_cleanup_task.py tests/unit/admin/scheduler/test_task_base.py -q (28 passed)
• Passed: uvx ruff check rock/admin/scheduler/tasks/image_cleanup_task.py tests/unit/admin/scheduler/test_image_cleanup_task.py
• Passed: uvx ruff format --check rock/admin/scheduler/tasks/image_cleanup_task.py tests/unit/admin/scheduler/test_image_cleanup_task.py