Add vigilar/alerts/sender.py with _CONTENT_MAP for human-readable push
notification titles/bodies, build_notification(), and send_alert() which
retrieves VAPID key, iterates push subscriptions, calls pywebpush, and
logs results to alert_log with auto-cleanup of expired (410) endpoints.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
