Keep Unauthenticated Server-Side Request Forgery via POST /providers/healthcheck

Keep (commit 91c75e0) contains a server-side request forgery vulnerability that allows unauthenticated attackers to make the backend issue arbitrary HTTP requests by supplying attacker-controlled host values to the unprotected healthcheck endpoint. Attackers can send a crafted JSON payload with a malicious host parameter to cause the backend to issue outbound requests to internal services or cloud metadata endpoints, enabling theft of cloud credentials and internal network reconnaissance.