refactor(layout): consolidate backend codebase into backend/ package and clean root directory

- Moved cmd/, core/, plugins/, pkg/, downstream/, and main.go into backend/ directory
- Batch updated all Go source files to import github.com/Rain-kl/Wavelet/backend/...
- Updated Makefile, scripts/swagger.sh, architecture guards, and platform skills
- Passed all quality gates (100% tests, 0 lint issues, clean build)

backend/plugins/domain/admin/plugin_test.go

@@ -0,0 +1,41 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package admin_test
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/Rain-kl/Wavelet/backend/core"
+	"github.com/Rain-kl/Wavelet/backend/plugins/domain/admin"
+)
+
+func TestAdminPluginUnit(t *testing.T) {
+	ctx := core.NewContext(context.Background())
+	p := admin.New()
+	assert.Equal(t, "admin", p.Name())
+	assert.Equal(t, "1.0.0", p.Manifest().Version)
+	require.NoError(t, p.Apply(ctx))
+
+	// Verify routes
+	routes := ctx.Router().Routes()
+	assert.NotEmpty(t, routes)
+
+	// Verify tasks
+	_, ok := ctx.Tasks().Get("admin:system_cleanup")
+	require.True(t, ok)
+
+	// Verify schedules
+	sched, ok := ctx.Schedules().Get("admin:system_cleanup")
+	require.True(t, ok)
+	assert.Equal(t, "0 4 * * *", sched.Spec)
+
+	// Verify settings
+	setting, ok := ctx.Settings().Get("admin.system_cleanup_cron")
+	require.True(t, ok)
+	assert.Equal(t, "0 4 * * *", setting.Default)
+}