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/message_gateway/plugin_test.go

@@ -0,0 +1,46 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package message_gateway_test
+
+import (
+	"context"
+	"io/fs"
+	"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/message_gateway"
+)
+
+func TestMessageGatewayPluginUnit(t *testing.T) {
+	ctx := core.NewContext(context.Background())
+	p := message_gateway.New()
+	assert.Equal(t, "message_gateway", p.Name())
+	assert.Equal(t, "1.0.0", p.Manifest().Version)
+	require.NoError(t, p.Apply(ctx))
+
+	// Verify migrations
+	entry, ok := ctx.Migrations().Get("message_gateway")
+	require.True(t, ok)
+	entries, err := fs.ReadDir(entry.FS, entry.Dir)
+	require.NoError(t, err)
+	assert.NotEmpty(t, entries)
+
+	// Verify tasks
+	task, ok := ctx.Tasks().Get("message_gateway:push_notification")
+	require.True(t, ok)
+	assert.Equal(t, 3, task.Retry)
+
+	// Verify schedules
+	sched, ok := ctx.Schedules().Get("message_gateway:cleanup_pairing_codes")
+	require.True(t, ok)
+	assert.Equal(t, "*/10 * * * *", sched.Spec)
+
+	// Verify settings
+	setting, ok := ctx.Settings().Get("message_gateway.max_bindings_per_user")
+	require.True(t, ok)
+	assert.Equal(t, 5, setting.Default)
+}