feat(obs): add operator event metrics

[typelicious]

What changed

• adds persistent operator-event metrics for update checks and helper-driven auto-update attempts
• adds GET /api/operator-events and operator-action breakdowns in /api/stats
• marks update-check helper calls with operator headers so the runtime can trace who triggered them

Why

• gives operators visibility into release-check activity without mixing it into normal request traces
• creates a clean observability seam for future scheduled update runs

How verified

• python3 -m compileall foundrygate tests
• PYTHONPATH=. ./.venv-check-313/bin/pytest -q tests/test_metrics_traces.py tests/test_updates.py tests/test_config.py
• ./.venv-check-313/bin/ruff check foundrygate/metrics.py foundrygate/main.py tests/test_metrics_traces.py
• ./.venv-check-313/bin/ruff format --check foundrygate/metrics.py foundrygate/main.py tests/test_metrics_traces.py
• bash -n scripts/foundrygate-update-check scripts/foundrygate-auto-update
• /usr/bin/git diff --check