test: add failing test for project background delete with read-only access

Proves that a user with read-only access to a project can delete its
background image. The test expects a 403 Forbidden but the operation
proceeds because RemoveProjectBackground only checks CanRead.

Adds fixture entry giving user 15 read-only access to project 35
(which has a background_file_id).

Ref: GHSA-564f-wx8x-878h

pkg/db/fixtures/users_projects.yml

@@ -112,3 +112,9 @@
   permission: 0
   updated: 2018-12-02 15:13:12
   created: 2018-12-01 15:13:12
+- id: 20
+  user_id: 15
+  project_id: 35
+  permission: 0
+  updated: 2018-12-02 15:13:12
+  created: 2018-12-01 15:13:12

pkg/webtests/background_test.go

@@ -0,0 +1,47 @@
+// Vikunja is a to-do list application to facilitate your life.
+// Copyright 2018-present Vikunja and contributors. All rights reserved.
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package webtests
+
+import (
+	"net/http"
+	"testing"
+
+	bgHandler "code.vikunja.io/api/pkg/modules/background/handler"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestProjectBackgroundDeletePermission(t *testing.T) {
+	t.Run("Read-only user cannot delete project background", func(t *testing.T) {
+		// testuser15 has read-only access (permission: 0) to project 35,
+		// which has background_file_id: 1.
+		// Deleting the background should require write access.
+		_, err := newTestRequestWithUser(
+			t,
+			http.MethodDelete,
+			bgHandler.RemoveProjectBackground,
+			&testuser15,
+			"",
+			nil,
+			map[string]string{"project": "35"},
+		)
+
+		// Should be forbidden for a read-only user
+		assert.Error(t, err)
+		assert.Equal(t, http.StatusForbidden, getHTTPErrorCode(err))
+	})
+}