CVE-2026-30242: Plane has SSRF via Incomplete IP Validation in Webhook URL Serializer

The webhook URL validation in plane/app/serializers/webhook.py only checks ip.is_loopback, allowing attackers with workspace ADMIN role to create webhooks pointing to private/internal network addresses (10.x.x.x, 172.16.x.x, 192.168.x.x, 169.254.169.254, etc.). When webhook events fire, the server makes requests to these internal addresses and stores the response — enabling SSRF with full response read-back.