Performs an SSH key response action (remove or deduplicate) across all hosts and users where the key is deployed. Requires the responder role. Creates one order per affected host, each containing one action per affected user. Can be disabled server-wide with SF_DISABLE_RESPONSES and at the node level with SF_NODE_DISABLE_RESPONSES. For 'remove' actions, a last-key protection safety check denies the request with HTTP 409 if any affected host would be left with zero SSH keys known to Sandfly; set allow_last_key_removal=true in the request body to override.