feat(ops): add update alert severity

[typelicious]

What changed

• extend GET /api/update with update_type, alert_level, and recommended_action
• classify available releases as patch, minor, major, or current
• surface stronger update signals in the dashboard release card
• document the stronger alert surface in README, roadmap, and changelog
• add unit coverage for update classification and alert mapping

Why

The existing update check answered only whether a newer release existed. Operators need a stronger signal that tells them how significant the update is and what action is recommended without interpreting version numbers manually.

How verified

• PYTHONPATH=. ./.venv-check-313/bin/pytest -q tests/test_updates.py
• ./.venv-check-313/bin/ruff check foundrygate/updates.py foundrygate/main.py tests/test_updates.py
• /usr/bin/git diff --check