ci(alerts): add manual Discord alert test mode

[JSONbored]

Summary

• adds a manual alert-test mode to the AIO Fleet Control Plane workflow
• keeps Discord webhook delivery behind the existing AIO_FLEET_ALERT_WEBHOOK_URL repo secret

What changed

• adds alert-test to the workflow_dispatch mode options
• adds a Test alert webhook step scoped only to the alert webhook secret and run URL
• verifies the step runs alert doctor and one real alert test without dry-run

Why

• lets us validate Discord alert delivery from GitHub Actions without exposing or committing the webhook URL

Validation

• gh secret list --repo JSONbored/aio-fleet
• .venv/bin/python -m pytest tests/test_control_plane_workflow.py tests/test_alerts.py -q

Notes

• does not change the existing secret value or name
• no package, image, or app repo release needed