feat: add FTP and SFTP upload support

internal/infrastructure/ftp/uploader_test.go

@@ -0,0 +1,34 @@
+package ftp
+
+import "testing"
+
+func TestValidateRemoteRelativePath(t *testing.T) {
+	valid, err := validateRemoteRelativePath("snapgo/2026/07/image.png")
+	if err != nil {
+		t.Fatalf("valid path rejected: %v", err)
+	}
+	if valid != "snapgo/2026/07/image.png" {
+		t.Fatalf("unexpected normalized path %q", valid)
+	}
+
+	invalid := []string{
+		"",
+		"/absolute/image.png",
+		"~/image.png",
+		"../image.png",
+		"snapgo/../../image.png",
+		"snapgo\\image.png",
+		"snapgo/image.png\nnext",
+	}
+	for _, value := range invalid {
+		if _, err := validateRemoteRelativePath(value); err == nil {
+			t.Errorf("expected %q to be rejected", value)
+		}
+	}
+}
+
+func TestProbeRemotePathRejectsTraversalPrefix(t *testing.T) {
+	if _, err := probeRemotePath("../../outside"); err == nil {
+		t.Fatal("expected traversal prefix to fail")
+	}
+}