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/pkg/util/uuid.go

@@ -0,0 +1,29 @@
+// Copyright 2025 linux.do
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package util
+
+import (
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/hex"
+	"io"
+
+	"github.com/google/uuid"
+)
+
+// uniqueIDBytes 生成唯一 ID 所需的随机字节长度
+const uniqueIDBytes = 32
+
+// GenerateUniqueIDSimple 生成 64 位唯一标识符
+func GenerateUniqueIDSimple() string {
+	randomBytes := make([]byte, uniqueIDBytes)
+	if _, err := io.ReadFull(rand.Reader, randomBytes); err != nil {
+		// 如果随机数生成失败，使用 UUID 作为后备
+		uuidBytes := []byte(uuid.NewString())
+		hash := sha256.Sum256(uuidBytes)
+		copy(randomBytes, hash[:])
+	}
+	return hex.EncodeToString(randomBytes)
+}