feat: add FTP and SFTP upload support

internal/infrastructure/ssh/sftp_uploader_test.go

@@ -0,0 +1,28 @@
+package ssh
+
+import "testing"
+
+func TestValidateSFTPRemotePathRejectsTraversal(t *testing.T) {
+	if got, err := validateSFTPRemotePath("snapgo/2026/image.png"); err != nil || got != "snapgo/2026/image.png" {
+		t.Fatalf("valid path: got %q err=%v", got, err)
+	}
+
+	for _, value := range []string{
+		"",
+		"/absolute/image.png",
+		"~/image.png",
+		"../image.png",
+		"snapgo/../../image.png",
+		"snapgo\\image.png",
+	} {
+		if _, err := validateSFTPRemotePath(value); err == nil {
+			t.Errorf("expected %q to be rejected", value)
+		}
+	}
+}
+
+func TestSFTPProbePathRejectsTraversalPrefix(t *testing.T) {
+	if _, err := sftpProbePath("../../outside"); err == nil {
+		t.Fatal("expected traversal prefix to fail")
+	}
+}