docs: expose notification setting value literal

[subzeroid]

Summary

• Add NotificationSettingValue = Union[SETTING_VALUE, MUTE_ALL] for the low-level notification helper.
• Annotate notification_settings(..., setting_value=...) with the public alias.
• Document notification option aliases and cover signatures/docs with regression tests.

Test plan

• .venv/bin/ruff check .
• .venv/bin/mkdocs build --strict
• ./scripts/check-mypy-baseline.sh
• .venv/bin/python -m pytest tests/regression -q