Add SSH-managed node registry with connection testing and reauth.

Register hosts under Containers/VMs/Docker with encrypted key storage, and require re-authentication for sensitive account changes.

service/internal/auth/ssh_connect_test.go

@@ -0,0 +1,19 @@
+package auth
+
+import (
+	"testing"
+)
+
+func TestTestSSHConnectionRejectsEmptyHost(t *testing.T) {
+	err := TestSSHConnection("", "clustercanvas", "not-a-key", "")
+	if err == nil {
+		t.Fatal("expected error")
+	}
+}
+
+func TestTestSSHConnectionRejectsInvalidKey(t *testing.T) {
+	err := TestSSHConnection("127.0.0.1", "clustercanvas", "not-a-key", "")
+	if err == nil {
+		t.Fatal("expected error")
+	}
+}