Expose runner queued job counts

[msaroufim]

Summary

• add runner queue status reporting for Modal and GitHub launchers
• expose runner_queue in background submission responses and via GET /runner_queue/{gpu_type}
• include focused tests for Modal, GitHub, backend routing, and API response shape

Validation

• uv run ruff check --select I,F src/libkernelbot/launchers/launcher.py src/libkernelbot/launchers/modal.py src/libkernelbot/launchers/github.py src/libkernelbot/backend.py src/libkernelbot/launchers/init.py src/kernelbot/api/main.py tests/test_backend.py tests/test_github.py tests/test_modal.py tests/test_admin_api.py
• uv run pytest tests/test_backend.py::test_get_runner_queue_status_routes_to_registered_launcher tests/test_backend.py::test_get_runner_queue_status_unknown_gpu tests/test_github.py::test_github_queue_status_counts_queued_workflow_runs tests/test_github.py::test_github_queue_status_reports_unavailable_on_error tests/test_modal.py::test_modal_queue_status_uses_function_stats tests/test_modal.py::test_modal_queue_status_reports_unavailable_on_error tests/test_admin_api.py::TestRunnerQueue::test_get_runner_queue

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
src/libkernelbot
backend.py					70
utils.py
Project Total

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