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/pairing_test.go

@@ -0,0 +1,33 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package message_gateway
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestGenerateCode_AlphabetAndLength(t *testing.T) {
+	code, err := GenerateCode()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(code) != 8 {
+		t.Fatalf("len=%d", len(code))
+	}
+	for _, r := range code {
+		if !strings.ContainsRune(CodeAlphabet, r) {
+			t.Fatalf("bad rune %q", r)
+		}
+	}
+}
+
+func TestNormalizeAndFormat(t *testing.T) {
+	if got := NormalizeCode("ab-cd-ef-gh"); got != "ABCDEFGH" {
+		t.Fatalf("got %q", got)
+	}
+	if got := FormatCode("ABCDEFGH"); got != "ABCD-EFGH" {
+		t.Fatalf("got %q", got)
+	}
+}