feat(audit): sanitize stored upstream responses

backend/internal/card/validation_test.go

@@ -53,3 +53,35 @@ func TestValidate(t *testing.T) {
 		})
 	}
 }
+
+func TestMask(t *testing.T) {
+	tests := []struct {
+		name   string
+		number string
+		want   string
+	}{
+		{
+			name:   "valid card number",
+			number: "6037997512345678",
+			want:   "603799******5678",
+		},
+		{
+			name:   "invalid card number",
+			number: "603799",
+			want:   "",
+		},
+		{
+			name:   "non numeric card number",
+			number: "603799ABC2345678",
+			want:   "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := Mask(tt.number); got != tt.want {
+				t.Errorf("Mask() = %q, want %q", got, tt.want)
+			}
+		})
+	}
+}