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/message_gateway_test.go

@@ -0,0 +1,30 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package message_gateway_test
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/Rain-kl/Wavelet/backend/pkg/testhelper"
+	"github.com/Rain-kl/Wavelet/backend/plugins/domain/message_gateway"
+)
+
+func TestUpsertPairingCode_ReusesUnexpired(t *testing.T) {
+	_, _, cleanup := testhelper.SetupTestEnvironment(t)
+	defer cleanup()
+	ctx := context.Background()
+	first, err := message_gateway.UpsertPairingCode(ctx, 1, "tg-1", "ABCD1234", time.Now().Add(15*time.Minute))
+	if err != nil {
+		t.Fatal(err)
+	}
+	second, err := message_gateway.UpsertPairingCode(ctx, 1, "tg-1", "ZZZZ9999", time.Now().Add(15*time.Minute))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if first.Code != second.Code || first.Code != "ABCD1234" {
+		t.Fatalf("reuse failed: %+v %+v", first, second)
+	}
+}