fix(alerts): send webhook requests with user agent

[JSONbored]

Summary

• adds an explicit aio-fleet User-Agent header to webhook deliveries

What changed

• sets User-Agent on outbound webhook POST requests
• extends alert tests to assert the header is present for generic and Discord webhook payloads

Why

• Discord accepted the webhook endpoint but rejected aio-fleet's Python client POSTs with HTTP 403 without an explicit User-Agent

Validation

• .venv/bin/python -m pytest tests/test_alerts.py tests/test_control_plane_workflow.py -q

Notes

• does not alter webhook URLs or commit any secret value