The webhook dispatcher validated webhook.url before posting but called
requests.post() without allow_redirects=False, so a webhook destination
could return a 3xx redirect to an internal address (cloud metadata,
internal services) and have the worker fetch it and persist the
response body to webhook_logs, readable back via the webhook-logs API.
Pass allow_redirects=False so the original validate_url() guard is
authoritative. Matches the pattern already used by safe_get() in
work_item_link_task.py and the behavior of GitHub/Stripe/Slack webhooks.
